nullable first step

Capnp.Net.Runtime/AnyPointer.cs

@@ -1,4 +1,5 @@
-namespace Capnp
+#nullable enable
+namespace Capnp
 {
     /// <summary>
     /// Generic <see cref="ICapnpSerializable"/> implementation, based on a wrapper around <see cref="DeserializerState"/>.
@@ -29,3 +30,4 @@
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Capnp.Net.Runtime.csproj

@@ -1,9 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netcoreapp2.1;netcoreapp3.0</TargetFrameworks>
     <RootNamespace>Capnp</RootNamespace>
-    <LangVersion>7.2</LangVersion>
+    <LangVersion>8.0</LangVersion>
     <AssemblyName>Capnp.Net.Runtime</AssemblyName>
     <PackageId>Capnp.Net.Runtime</PackageId>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>

Capnp.Net.Runtime/CapnpSerializable.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Reflection;
 using System.Runtime.CompilerServices;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -10,15 +11,15 @@ namespace Capnp
     /// </summary>
     public static class CapnpSerializable
     {
-        interface IConstructibleFromDeserializerState<out T>
+        interface IConstructibleFromDeserializerState
         {
-            T Create(DeserializerState state);
+            object? Create(DeserializerState state);
         }
 
-        class FromStruct<T>: IConstructibleFromDeserializerState<T>
+        class FromStruct<T>: IConstructibleFromDeserializerState
             where T : ICapnpSerializable, new()
         {
-            public T Create(DeserializerState state)
+            public object Create(DeserializerState state)
             {
                 var result = new T();
                 if (state.Kind != ObjectKind.Nil)
@@ -29,7 +30,7 @@ namespace Capnp
             }
         }
 
-        class FromList<T>: IConstructibleFromDeserializerState<IReadOnlyList<T>>
+        class FromList<T>: IConstructibleFromDeserializerState
             where T: class
         {
             readonly Func<DeserializerState, T> _elementSerializer;
@@ -39,23 +40,23 @@ namespace Capnp
                 _elementSerializer = (Func<DeserializerState, T>)GetSerializer(typeof(T));
 
             }
-            public IReadOnlyList<T> Create(DeserializerState state)
+            public object Create(DeserializerState state)
             {
                 return state.RequireList().Cast(_elementSerializer);
             }
         }
 
-        class FromCapability<T>: IConstructibleFromDeserializerState<T>
+        class FromCapability<T>: IConstructibleFromDeserializerState
             where T: class
         {
-            public T Create(DeserializerState state)
+            public object? Create(DeserializerState state)
             {
                 return state.RequireCap<T>();
             }
         }
 
-        static readonly ConditionalWeakTable<Type, Func<DeserializerState, object>> _typeMap =
+        static readonly ConditionalWeakTable<Type, Func<DeserializerState, object?>> _typeMap =
-            new ConditionalWeakTable<Type, Func<DeserializerState, object>>();
+            new ConditionalWeakTable<Type, Func<DeserializerState, object?>>();
 
         static CapnpSerializable()
         {
@@ -73,14 +74,14 @@ namespace Capnp
             _typeMap.Add(typeof(IReadOnlyList<double>), d => d.RequireList().CastDouble());
         }
 
-        static Func<DeserializerState, object> CreateSerializer(Type type)
+        static Func<DeserializerState, object?> CreateSerializer(Type type)
         {
             if (typeof(ICapnpSerializable).IsAssignableFrom(type))
             {
                 try
                 {
-                    return ((IConstructibleFromDeserializerState<object>)
+                    return ((IConstructibleFromDeserializerState)
-                        Activator.CreateInstance(typeof(FromStruct<>).MakeGenericType(type))).Create;
+                        Activator.CreateInstance(typeof(FromStruct<>).MakeGenericType(type))!).Create;
                 }
                 catch (Exception ex)
                 {
@@ -94,12 +95,12 @@ namespace Capnp
                 try
                 {
                     var elementType = type.GetGenericArguments()[0];
-                    return ((IConstructibleFromDeserializerState<object>)
+                    return ((IConstructibleFromDeserializerState)
-                        Activator.CreateInstance(typeof(FromList<>).MakeGenericType(elementType))).Create;
+                        Activator.CreateInstance(typeof(FromList<>).MakeGenericType(elementType))!).Create;
                 }
                 catch (TargetInvocationException ex)
                 {
-                    throw ex.InnerException;
+                    throw ex.InnerException!;
                 }
                 catch (Exception ex)
                 {
@@ -123,8 +124,8 @@ namespace Capnp
 
                 try
                 {
-                    return ((IConstructibleFromDeserializerState<object>)
+                    return ((IConstructibleFromDeserializerState)
-                        Activator.CreateInstance(typeof(FromCapability<>).MakeGenericType(type))).Create;
+                        Activator.CreateInstance(typeof(FromCapability<>).MakeGenericType(type))!).Create;
                 }
                 catch (Exception ex)
                 {
@@ -135,7 +136,7 @@ namespace Capnp
             }
         }
 
-        static Func<DeserializerState, object> GetSerializer(Type type)
+        static Func<DeserializerState, object?> GetSerializer(Type type)
         {
             return _typeMap.GetValue(type, CreateSerializer);
         }
@@ -164,10 +165,11 @@ namespace Capnp
         /// </typeparam>
         /// <param name="state"></param>
         /// <returns></returns>
-        public static T Create<T>(DeserializerState state)
+        public static T? Create<T>(DeserializerState state)
             where T: class
         {
-            return (T)GetSerializer(typeof(T))(state);
+            return (T?)GetSerializer(typeof(T))(state);
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/DeserializationException.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -23,3 +24,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/DeserializerState.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -47,7 +49,7 @@ namespace Capnp
         /// <summary>
         /// The capabilities imported from the capability table. Only valid in RPC context.
         /// </summary>
-        public IList<Rpc.ConsumedCapability> Caps { get; set; }
+        public IList<Rpc.ConsumedCapability?>? Caps { get; set; }
         /// <summary>
         /// Current segment (essentially Segments[CurrentSegmentIndex]
         /// </summary>
@@ -83,11 +85,16 @@ namespace Capnp
         /// The conversion is cheap, since it does not involve copying any payload.
         /// </summary>
         /// <param name="state">The serializer state to be converted</param>
+        /// <exception cref="ArgumentNullException"><paramref name="state"/> is null</exception>
+        /// <exception cref="InvalidOperationException"><paramref name="state"/> is not bound to a MessageBuilder</exception>
         public static implicit operator DeserializerState(SerializerState state)
         {
             if (state == null)
                 throw new ArgumentNullException(nameof(state));
 
+            if (state.MsgBuilder == null)
+                throw new InvalidOperationException("state is not bound to a MessageBuilder");
+            
             switch (state.Kind)
             {
                 case ObjectKind.ListOfBits:
@@ -100,7 +107,7 @@ namespace Capnp
                 case ObjectKind.ListOfStructs:
                 case ObjectKind.Nil:
                 case ObjectKind.Struct:
-                    return new DeserializerState(state.Allocator.Segments)
+                    return new DeserializerState(state.Allocator!.Segments)
                     {
                         CurrentSegmentIndex = state.SegmentIndex,
                         Offset = state.Offset,
@@ -112,7 +119,7 @@ namespace Capnp
                     };
 
                 case ObjectKind.Capability:
-                    return new DeserializerState(state.Allocator.Segments)
+                    return new DeserializerState(state.Allocator!.Segments)
                     {
                         Kind = ObjectKind.Capability,
                         Caps = state.Caps,
@@ -376,11 +383,11 @@ namespace Capnp
         /// the capability table. Does not mutate this state.
         /// </summary>
         /// <param name="offset">Offset relative to this.Offset within current segment</param>
-        /// <returns>the low-level capability object</returns>
+        /// <returns>the low-level capability object, or null if it is a null pointer</returns>
         /// <exception cref="IndexOutOfRangeException">offset negative or out of range</exception>
         /// <exception cref="InvalidOperationException">capability table not set</exception>
         /// <exception cref="Rpc.RpcException">not a capability pointer or invalid capability index</exception>
-        internal Rpc.ConsumedCapability DecodeCapPointer(int offset)
+        internal Rpc.ConsumedCapability? DecodeCapPointer(int offset)
         {
             if (offset < 0)
             {
@@ -490,7 +497,7 @@ namespace Capnp
             return state;
         }
 
-        internal Rpc.ConsumedCapability StructReadRawCap(int index)
+        internal Rpc.ConsumedCapability? StructReadRawCap(int index)
         {
             if (Kind != ObjectKind.Struct && Kind != ObjectKind.Nil)
                 throw new InvalidOperationException("Allowed on structs only");
@@ -567,7 +574,7 @@ namespace Capnp
         /// <exception cref="IndexOutOfRangeException">negative index</exception>
         /// <exception cref="DeserializationException">state does not represent a struct, invalid pointer,
         /// non-list-of-bytes pointer, traversal limit exceeded</exception>
-        public string ReadText(int index, string defaultText = null)
+        public string? ReadText(int index, string? defaultText = null)
         {
             return StructReadPointer(index).RequireList().CastText() ?? defaultText;
         }
@@ -646,7 +653,7 @@ namespace Capnp
         /// <exception cref="IndexOutOfRangeException">negative index</exception>
         /// <exception cref="DeserializationException">state does not represent a struct, invalid pointer,
         /// non-capability pointer, traversal limit exceeded</exception>
-        public T ReadCap<T>(int index,
+        public T? ReadCap<T>(int index,
             [System.Runtime.CompilerServices.CallerMemberName] string memberName = "",
             [System.Runtime.CompilerServices.CallerFilePath] string sourceFilePath = "",
             [System.Runtime.CompilerServices.CallerLineNumber] int sourceLineNumber = 0) where T: class
@@ -677,7 +684,7 @@ namespace Capnp
         /// <returns>capability instance or null if pointer was null</returns>
         /// <exception cref="IndexOutOfRangeException">negative index</exception>
         /// <exception cref="DeserializationException">state does not represent a capability</exception>
-        public T RequireCap<T>() where T: class
+        public T? RequireCap<T>() where T: class
         {
             if (Kind == ObjectKind.Nil)
                 return null;
@@ -685,7 +692,11 @@ namespace Capnp
             if (Kind != ObjectKind.Capability)
                 throw new DeserializationException("Expected a capability");
 
+            if (Caps == null)
+                throw new InvalidOperationException("Capability table not set. This is a bug.");
+
             return Rpc.CapabilityReflection.CreateProxy<T>(Caps[(int)CapabilityIndex]) as T;
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/DynamicSerializerState.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -226,3 +227,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/EmptyList.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -36,3 +37,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/EmptyListDeserializer.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -86,3 +87,4 @@ namespace Capnp
         public override IReadOnlyList<ushort> CastUShort() => new EmptyList<ushort>();
     }
 }
+#nullable restore

Capnp.Net.Runtime/FramePump.cs

@@ -9,6 +9,7 @@ using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -22,7 +23,7 @@ namespace Capnp
 
         int _disposing;
         readonly Stream _stream;
-        readonly BinaryWriter _writer;
+        readonly BinaryWriter? _writer;
         readonly object _writeLock = new object();
         readonly List<IFrameTracer> _tracers = new List<IFrameTracer>();
 
@@ -62,7 +63,7 @@ namespace Capnp
         /// <summary>
         /// Event handler for frame reception.
         /// </summary>
-        public event Action<WireFrame> FrameReceived;
+        public event Action<WireFrame>? FrameReceived;
 
         /// <summary>
         /// Sends a message over the stream.
@@ -196,3 +197,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/FrameTracing/IFrameTracer.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp.FrameTracing
 {
     /// <summary>
@@ -31,3 +32,4 @@ namespace Capnp.FrameTracing
         void TraceFrame(FrameDirection direction, WireFrame frame);
     }
 }
+#nullable restore

Capnp.Net.Runtime/FrameTracing/RpcFrameTracer.cs

@@ -6,6 +6,7 @@ using System.IO;
 using System.Linq;
 using System.Threading;
 
+#nullable enable
 namespace Capnp.FrameTracing
 {
     /// <summary>
@@ -245,3 +246,4 @@ namespace Capnp.FrameTracing
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Framing.cs

@@ -3,6 +3,7 @@ using System.IO;
 using System.Runtime.InteropServices;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -107,3 +108,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ICapnpSerializable.cs

@@ -1,4 +1,5 @@
-namespace Capnp
+#nullable enable
+namespace Capnp
 {
     /// <summary>
     /// This interface is intended to be implemented by schema-generated domain classes which support deserialization from
@@ -19,3 +20,4 @@
         void Deserialize(DeserializerState state);
     }
 }
+#nullable restore

Capnp.Net.Runtime/ISegmentAllocator.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -27,3 +28,4 @@ namespace Capnp
         bool Allocate(uint nwords, uint preferredSegment, out SegmentSlice slice, bool forcePreferredSegment);
     }
 }
+#nullable restore

Capnp.Net.Runtime/IStructDeserializer.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -20,3 +21,4 @@ namespace Capnp
         ulong StructReadData(ulong bitOffset, int bitCount);
     }
 }
+#nullable restore

Capnp.Net.Runtime/IStructSerializer.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -25,3 +26,4 @@ namespace Capnp
         Span<ulong> StructDataSection { get; }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListDeserializer.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -10,7 +11,7 @@ namespace Capnp
     {
         static class GenericCasts<T>
         {
-            public static Func<ListDeserializer, T> CastFunc;
+            public static Func<ListDeserializer, T>? CastFunc;
         }
 
         static ListDeserializer()
@@ -401,3 +402,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListKind.cs

@@ -1,4 +1,5 @@
-namespace Capnp
+#nullable enable
+namespace Capnp
 {
     /// <summary>
     /// Enumerates the list element categories which are defined by Cap'n Proto.
@@ -46,3 +47,4 @@
         ListOfStructs = 7
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfBitsDeserializer.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -77,3 +78,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfBitsSerializer.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -96,3 +97,4 @@ namespace Capnp
         IEnumerator IEnumerable.GetEnumerator() => this.ToArray().GetEnumerator();
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfCapsDeserializer.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -29,7 +30,7 @@ namespace Capnp
                 if (index < 0 || index >= Count)
                     throw new IndexOutOfRangeException();
 
-                return Rpc.CapabilityReflection.CreateProxy<T>(State.DecodeCapPointer(index)) as T;
+                return (Rpc.CapabilityReflection.CreateProxy<T>(State.DecodeCapPointer(index)) as T)!;
             }
         }
 
@@ -70,3 +71,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfCapsSerializer.cs

@@ -1,8 +1,8 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -34,7 +34,7 @@ namespace Capnp
         /// <exception cref="IndexOutOfRangeException"><paramref name="index"/> is out of range.</exception>
         public T this[int index]
         {
-            get => Rpc.CapabilityReflection.CreateProxy<T>(DecodeCapPointer(index)) as T;
+            get => (Rpc.CapabilityReflection.CreateProxy<T>(DecodeCapPointer(index)) as T)!;
             set
             {
                 if (!IsAllocated)
@@ -109,3 +109,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfEmptyDeserializer.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -64,3 +65,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfEmptySerializer.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -31,3 +32,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfPointersDeserializer.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -89,3 +90,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfPointersSerializer.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -12,7 +13,7 @@ namespace Capnp
     /// <typeparam name="TS">SerializerState which represents the element type</typeparam>
     public class ListOfPointersSerializer<TS>:
         SerializerState,
-        IReadOnlyList<TS>
+        IReadOnlyList<TS?>
         where TS: SerializerState, new()
     {
         /// <summary>
@@ -51,7 +52,7 @@ namespace Capnp
         /// </summary>
         public int Count => ListElementCount;
 
-        IEnumerable<TS> Enumerate()
+        IEnumerable<TS?> Enumerate()
         {
             int count = Count;
 
@@ -64,7 +65,7 @@ namespace Capnp
         /// <summary>
         /// Implements <see cref="IEnumerable{TS}"/>.
         /// </summary>
-        public IEnumerator<TS> GetEnumerator()
+        public IEnumerator<TS?> GetEnumerator()
         {
             return Enumerate().GetEnumerator();
         }
@@ -115,4 +116,4 @@ namespace Capnp
         }
     }
 }
-
+#nullable enable

Capnp.Net.Runtime/ListOfPrimitivesDeserializer.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -232,3 +233,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfPrimitivesSerializer.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -94,3 +95,4 @@ namespace Capnp
         IEnumerator IEnumerable.GetEnumerator() => Data.ToArray().GetEnumerator();
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfStructsDeserializer.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -79,3 +80,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfStructsSerializer.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -93,3 +94,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ListOfTextSerializer.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -9,7 +10,7 @@ namespace Capnp
     /// </summary>
     public class ListOfTextSerializer :
         SerializerState,
-        IReadOnlyList<string>
+        IReadOnlyList<string?>
     {
         /// <summary>
         /// Gets or sets the text at given index. Once an element is set, it cannot be overwritten.
@@ -18,7 +19,7 @@ namespace Capnp
         /// <exception cref="InvalidOperationException">List is not initialized</exception>
         /// <exception cref="IndexOutOfRangeException"><paramref name="index"/> is out of range.</exception>
         /// <exception cref="ArgumentOutOfRangeException">UTF-8 encoding exceeds 2^29-2 bytes</exception>
-        public string this[int index]
+        public string? this[int index]
         {
             get
             {
@@ -47,7 +48,7 @@ namespace Capnp
         /// </summary>
         public int Count => ListElementCount;
 
-        IEnumerable<string> Enumerate()
+        IEnumerable<string?> Enumerate()
         {
             int count = Count;
 
@@ -60,7 +61,7 @@ namespace Capnp
         /// <summary>
         /// Implementation of <see cref="IEnumerable{String}"/>/>
         /// </summary>
-        public IEnumerator<string> GetEnumerator()
+        public IEnumerator<string?> GetEnumerator()
         {
             return Enumerate().GetEnumerator();
         }
@@ -110,3 +111,4 @@ namespace Capnp
     }
 }
 
+#nullable restore

Capnp.Net.Runtime/Logging.cs

@@ -1,6 +1,7 @@
 using System;
 using Microsoft.Extensions.Logging;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -21,3 +22,4 @@ namespace Capnp
         public static ILogger CreateLogger<T>() => LoggerFactory.CreateLogger<T>();
     }
 }
+#nullable restore

Capnp.Net.Runtime/MessageBuilder.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -12,7 +13,7 @@ namespace Capnp
     {
         readonly ISegmentAllocator _allocator;
         readonly DynamicSerializerState _rootPtrBuilder;
-        List<Rpc.ConsumedCapability> _capTable;
+        List<Rpc.ConsumedCapability?>? _capTable;
 
         MessageBuilder(ISegmentAllocator allocator)
         {
@@ -56,10 +57,11 @@ namespace Capnp
         /// Gets or sets the root object. The root object must be set exactly once per message.
         /// Setting it manually is only required (and allowed) when it was created with <see cref="CreateObject{TS}"/>.
         /// </summary>
-        public SerializerState Root
+        /// <exception cref="ArgumentNullException">Attempt to set null reference</exception>
+        public SerializerState? Root
         {
             get => _rootPtrBuilder.TryGetPointer(0);
-            set => _rootPtrBuilder.Link(0, value);
+            set => _rootPtrBuilder.Link(0, value ?? throw new ArgumentNullException(nameof(value)));
         }
 
         /// <summary>
@@ -90,13 +92,14 @@ namespace Capnp
             if (_capTable != null)
                 throw new InvalidOperationException("Capability table was already initialized");
 
-            _capTable = new List<Rpc.ConsumedCapability>();
+            _capTable = new List<Rpc.ConsumedCapability?>();
         }
 
         /// <summary>
         /// Returns this message builder's segment allocator.
         /// </summary>
         public ISegmentAllocator Allocator => _allocator;
-        internal List<Rpc.ConsumedCapability> Caps => _capTable;
+        internal List<Rpc.ConsumedCapability?>? Caps => _capTable;
     }
 }
+#nullable restore

Capnp.Net.Runtime/ObjectKind.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -71,3 +72,4 @@ namespace Capnp
         Value = 16
     }
 }
+#nullable restore

Capnp.Net.Runtime/PrimitiveCoder.cs

@@ -2,13 +2,14 @@
 using System.Collections.Generic;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     class PrimitiveCoder
     {
         class Coder<T>
         {
-            public static Func<T, T, T> Fn { get; set; }
+            public static Func<T, T, T>? Fn { get; set; }
         }
 
         static PrimitiveCoder()
@@ -45,3 +46,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/ReadOnlyListExtensions.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -71,3 +72,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Reserializing.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -103,3 +104,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/AnswerOrCounterquestion.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// Helper struct to support tail calls
@@ -33,11 +34,12 @@
         /// <summary>
         /// SerializerState, if applicable
         /// </summary>
-        public SerializerState Answer => _obj as SerializerState;
+        public SerializerState? Answer => _obj as SerializerState;
 
         /// <summary>
         /// PendingQuestion, if applicable
         /// </summary>
-        public PendingQuestion Counterquestion => _obj as PendingQuestion;
+        public PendingQuestion? Counterquestion => _obj as PendingQuestion;
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/BareProxy.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// Generic Proxy implementation which exposes the (usually protected) Call method.
@@ -33,7 +34,7 @@
         /// Constructs an instance and binds it to the given low-level capability.
         /// </summary>
         /// <param name="cap">low-level capability</param>
-        public BareProxy(ConsumedCapability cap): base(cap)
+        public BareProxy(ConsumedCapability? cap): base(cap)
         {
         }
 
@@ -50,3 +51,4 @@
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/CapabilityReflection.cs

@@ -3,6 +3,7 @@ using System.Diagnostics;
 using System.Linq;
 using System.Runtime.CompilerServices;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -110,7 +111,7 @@ namespace Capnp.Rpc
 
             return (SkeletonFactory)Activator.CreateInstance(
                 typeof(SkeletonFactory<>)
-                .MakeGenericType(skeletonClass));
+                .MakeGenericType(skeletonClass))!;
         }
 
         static SkeletonFactory GetSkeletonFactory(Type type)
@@ -190,7 +191,7 @@ namespace Capnp.Rpc
 
                         return (ProxyFactory)Activator.CreateInstance(
                             typeof(ProxyFactory<>)
-                            .MakeGenericType(proxyClass));
+                            .MakeGenericType(proxyClass))!;
                     }
                     else
                     {
@@ -263,7 +264,7 @@ namespace Capnp.Rpc
         /// <exception cref="System.Reflection.TargetInvocationException">Problem with instatiating the Proxy (constructor threw exception).</exception>
         /// <exception cref="MemberAccessException">Caller does not have permission to invoke the Proxy constructor.</exception>
         /// <exception cref="TypeLoadException">Problem with building the Proxy type, or problem with loading some dependent class.</exception>
-        public static Proxy CreateProxy<TInterface>(ConsumedCapability cap,
+        public static Proxy CreateProxy<TInterface>(ConsumedCapability? cap,
             [System.Runtime.CompilerServices.CallerMemberName] string memberName = "",
             [System.Runtime.CompilerServices.CallerFilePath] string sourceFilePath = "",
             [System.Runtime.CompilerServices.CallerLineNumber] int sourceLineNumber = 0)
@@ -286,3 +287,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/ConnectionState.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// State of an RPC connection
@@ -22,3 +23,4 @@
         Down
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/ConsumedCapability.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -18,7 +19,7 @@ namespace Capnp.Rpc
         /// </summary>
         protected abstract void ReleaseRemotely();
         internal abstract void Export(IRpcEndpoint endpoint, CapDescriptor.WRITER writer);
-        internal abstract void Freeze(out IRpcEndpoint boundEndpoint);
+        internal abstract void Freeze(out IRpcEndpoint? boundEndpoint);
         internal abstract void Unfreeze();
 
         internal abstract void AddRef();
@@ -31,3 +32,4 @@ namespace Capnp.Rpc
 #endif
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IConnection.cs

@@ -1,6 +1,7 @@
 using Capnp.FrameTracing;
 using System;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -58,3 +59,4 @@ namespace Capnp.Rpc
         void Close();
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IEndpoint.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// A uni-directional endpoint, used in conjunction with the <see cref="RpcEngine"/>.
@@ -16,3 +17,4 @@
         void Dismiss();
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IMonoSkeleton.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// A mono skeleton (as opposed to <see cref="PolySkeleton"/>) is a skeleton which implements one particular RPC interface.
@@ -11,3 +12,4 @@
         ulong InterfaceId { get; }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IPromisedAnswer.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -22,6 +23,7 @@ namespace Capnp.Rpc
         /// </summary>
         /// <param name="access">Path to the desired capability inside the result struct.</param>
         /// <returns>Pipelined low-level capability</returns>
-        ConsumedCapability Access(MemberAccessPath access);
+        ConsumedCapability? Access(MemberAccessPath access);
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IProvidedCapability.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
 
@@ -22,3 +23,4 @@ namespace Capnp.Rpc
             DeserializerState args, CancellationToken cancellationToken = default);
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IResolvingCapability.cs

@@ -1,5 +1,6 @@
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -13,3 +14,4 @@ namespace Capnp.Rpc
         Task<Proxy> WhenResolved { get; }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/IRpcEndpoint.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     internal interface IRpcEndpoint
@@ -19,3 +20,4 @@ namespace Capnp.Rpc
         void DeleteQuestion(PendingQuestion question);
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Impatient.cs

@@ -3,6 +3,7 @@ using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -11,7 +12,7 @@ namespace Capnp.Rpc
     public static class Impatient
     {
         static readonly ConditionalWeakTable<Task, IPromisedAnswer> _taskTable = new ConditionalWeakTable<Task, IPromisedAnswer>();
-        static readonly ThreadLocal<IRpcEndpoint> _askingEndpoint = new ThreadLocal<IRpcEndpoint>();
+        static readonly ThreadLocal<IRpcEndpoint?> _askingEndpoint = new ThreadLocal<IRpcEndpoint?>();
 
         /// <summary>
         /// Attaches a continuation to the given promise and registers the resulting task for pipelining.
@@ -51,7 +52,7 @@ namespace Capnp.Rpc
                     }
                     else if (rtask.IsFaulted)
                     {
-                        rtask = Task.FromException<T>(rtask.Exception.InnerException);
+                        rtask = Task.FromException<T>(rtask.Exception!.InnerException!);
                     }
                     else
                     {
@@ -86,7 +87,7 @@ namespace Capnp.Rpc
             return answer;
         }
 
-        internal static IPromisedAnswer TryGetAnswer(Task task)
+        internal static IPromisedAnswer? TryGetAnswer(Task task)
         {
             _taskTable.TryGetValue(task, out var answer);
             return answer;
@@ -102,10 +103,10 @@ namespace Capnp.Rpc
                     return proxy;
 
                 case null:
-                    return null;
+                    return CapabilityReflection.CreateProxy<T>(null);
             }
 
-            var skel = Skeleton.GetOrCreateSkeleton(item, false);
+            var skel = Skeleton.GetOrCreateSkeleton(item!, false);
             var localCap = LocalCapability.Create(skel);
             return CapabilityReflection.CreateProxy<T>(localCap);
         }
@@ -131,7 +132,7 @@ namespace Capnp.Rpc
             where TInterface : class
         {
             var lazyCap = new LazyCapability(AwaitProxy(task));
-            return CapabilityReflection.CreateProxy<TInterface>(lazyCap, memberName, sourceFilePath, sourceLineNumber) as TInterface;
+            return (CapabilityReflection.CreateProxy<TInterface>(lazyCap, memberName, sourceFilePath, sourceLineNumber) as TInterface)!;
         }
 
         static readonly MemberAccessPath Path_OneAndOnly = new MemberAccessPath(0U);
@@ -168,15 +169,15 @@ namespace Capnp.Rpc
                 }
 
                 var lazyCap = new LazyCapability(AwaitProxy(task));
-                return CapabilityReflection.CreateProxy<TInterface>(lazyCap) as TInterface;
+                return (CapabilityReflection.CreateProxy<TInterface>(lazyCap) as TInterface)!;
             }
             else
             {
-                return CapabilityReflection.CreateProxy<TInterface>(answer.Access(Path_OneAndOnly)) as TInterface;
+                return (CapabilityReflection.CreateProxy<TInterface>(answer.Access(Path_OneAndOnly)) as TInterface)!;
             }
         }
 
-        internal static IRpcEndpoint AskingEndpoint
+        internal static IRpcEndpoint? AskingEndpoint
         {
             get => _askingEndpoint.Value;
             set { _askingEndpoint.Value = value; }
@@ -252,3 +253,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/ImportedCapability.cs

@@ -1,5 +1,6 @@
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -52,3 +53,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Interception/CallContext.cs

@@ -5,6 +5,7 @@ using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc.Interception
 {
 
@@ -34,7 +35,7 @@ namespace Capnp.Rpc.Interception
                 return new Proxy(Access(access));
             }
 
-            public ConsumedCapability Access(MemberAccessPath access)
+            public ConsumedCapability? Access(MemberAccessPath access)
             {
                 if (_futureResult.Task.IsCompleted)
                 {
@@ -44,7 +45,7 @@ namespace Capnp.Rpc.Interception
                     }
                     catch (AggregateException exception)
                     {
-                        throw exception.InnerException;
+                        throw exception.InnerException!;
                     }
                 }
                 else
@@ -107,7 +108,7 @@ namespace Capnp.Rpc.Interception
         /// <summary>
         /// Input arguments
         /// </summary>
-        public SerializerState InArgs { get; set; }
+        public SerializerState? InArgs { get; set; }
 
         /// <summary>
         /// Output arguments ("return value")
@@ -117,7 +118,7 @@ namespace Capnp.Rpc.Interception
         /// <summary>
         /// Exception text, or null if there is no exception
         /// </summary>
-        public string Exception { get; set; }
+        public string? Exception { get; set; }
 
         /// <summary>
         /// Whether the call should return in canceled state to Alice (the original caller).
@@ -156,7 +157,7 @@ namespace Capnp.Rpc.Interception
         /// <item><description>null</description></item>
         /// </list>
         /// </summary>
-        public object Bob 
+        public object? Bob 
         {
             get => _bob;
             set
@@ -196,11 +197,11 @@ namespace Capnp.Rpc.Interception
             }
         }
 
-        internal Proxy BobProxy { get; private set; }
+        internal Proxy? BobProxy { get; private set; }
 
         readonly CensorCapability _censorCapability;
         PromisedAnswer _promisedAnswer;
-        object _bob;
+        object? _bob;
 
         internal IPromisedAnswer Answer => _promisedAnswer;
 
@@ -224,8 +225,13 @@ namespace Capnp.Rpc.Interception
             {
                 for (int i = 0; i < state.Caps.Count; i++)
                 {
-                    state.Caps[i] = policy.Attach(state.Caps[i]);
+                    var cap = state.Caps[i];
-                    state.Caps[i].AddRef();
+                    if (cap != null)
+                    {
+                        cap = policy.Attach(cap);
+                        state.Caps[i] = cap;
+                        cap.AddRef();
+                    }
                 }
             }
         }
@@ -236,8 +242,13 @@ namespace Capnp.Rpc.Interception
             {
                 for (int i = 0; i < state.Caps.Count; i++)
                 {
-                    state.Caps[i] = policy.Detach(state.Caps[i]);
+                    var cap = state.Caps[i];
-                    state.Caps[i].AddRef();
+                    if (cap != null)
+                    {
+                        cap = policy.Detach(cap);
+                        state.Caps[i] = cap;
+                        cap.AddRef();
+                    }
                 }
             }
         }
@@ -246,8 +257,12 @@ namespace Capnp.Rpc.Interception
         /// Intercepts all capabilies inside the input arguments
         /// </summary>
         /// <param name="policyOverride">Policy to use, or null to further use present policy</param>
-        public void InterceptInCaps(IInterceptionPolicy policyOverride = null)
+        /// <exception cref="InvalidOperationException">InArgs not set</exception>
+        public void InterceptInCaps(IInterceptionPolicy? policyOverride = null)
         {
+            if (InArgs == null)
+                throw new InvalidOperationException("InArgs not set");
+
             InterceptCaps(InArgs, policyOverride ?? _censorCapability.Policy);
         }
 
@@ -255,7 +270,7 @@ namespace Capnp.Rpc.Interception
         /// Intercepts all capabilies inside the output arguments
         /// </summary>
         /// <param name="policyOverride">Policy to use, or null to further use present policy</param>
-        public void InterceptOutCaps(IInterceptionPolicy policyOverride = null)
+        public void InterceptOutCaps(IInterceptionPolicy? policyOverride = null)
         {
             InterceptCaps(OutArgs, policyOverride ?? _censorCapability.Policy);
         }
@@ -264,8 +279,12 @@ namespace Capnp.Rpc.Interception
         /// Unintercepts all capabilies inside the input arguments
         /// </summary>
         /// <param name="policyOverride">Policy to remove, or null to remove present policy</param>
-        public void UninterceptInCaps(IInterceptionPolicy policyOverride = null)
+        /// <exception cref="InvalidOperationException">InArgs not set</exception>
+        public void UninterceptInCaps(IInterceptionPolicy? policyOverride = null)
         {
+            if (InArgs == null)
+                throw new InvalidOperationException("InArgs not set");
+
             UninterceptCaps(InArgs, policyOverride ?? _censorCapability.Policy);
         }
 
@@ -273,7 +292,7 @@ namespace Capnp.Rpc.Interception
         /// Unintercepts all capabilies inside the output arguments
         /// </summary>
         /// <param name="policyOverride">Policy to remove, or null to remove present policy</param>
-        public void UninterceptOutCaps(IInterceptionPolicy policyOverride = null)
+        public void UninterceptOutCaps(IInterceptionPolicy? policyOverride = null)
         {
             UninterceptCaps(OutArgs, policyOverride ?? _censorCapability.Policy);
         }
@@ -281,14 +300,16 @@ namespace Capnp.Rpc.Interception
         /// <summary>
         /// Forwards this intercepted call to the target capability ("Bob").
         /// </summary>
+        /// <exception cref="InvalidOperationException">Bob/InArgs not set</exception>
         public void ForwardToBob()
         {
             if (Bob == null)
-            {
                 throw new InvalidOperationException("Bob is null");
-            }
 
-            var answer = BobProxy.Call(InterfaceId, MethodId, InArgs.Rewrap<DynamicSerializerState>(), default, CancelToBob);
+            if (InArgs == null)
+                throw new InvalidOperationException("InArgs not set");
+
+            var answer = BobProxy!.Call(InterfaceId, MethodId, InArgs.Rewrap<DynamicSerializerState>(), default, CancelToBob);
 
             State = InterceptionState.ForwardedToBob;
 
@@ -336,3 +357,4 @@ namespace Capnp.Rpc.Interception
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Interception/CensorCapability.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc.Interception
+#nullable enable
+namespace Capnp.Rpc.Interception
 {
     class CensorCapability : RefCountingCapability
     {
@@ -32,7 +33,7 @@
             writer.SenderHosted = endpoint.AllocateExport(MyVine, out bool _);
         }
 
-        internal override void Freeze(out IRpcEndpoint boundEndpoint)
+        internal override void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             boundEndpoint = null;
         }
@@ -42,3 +43,4 @@
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Interception/IInterceptionPolicy.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp.Rpc.Interception
 {
     /// <summary>
@@ -21,3 +22,4 @@ namespace Capnp.Rpc.Interception
         void OnReturnFromBob(CallContext callContext);
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Interception/InterceptionState.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc.Interception
+#nullable enable
+namespace Capnp.Rpc.Interception
 {
     /// <summary>
     /// The state of an intercepted call from Alice to Bob.
@@ -26,3 +27,4 @@
         ReturnedToAlice
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Interception/Interceptor.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
+#nullable enable
 namespace Capnp.Rpc.Interception
 {
     /// <summary>
@@ -45,16 +46,16 @@ namespace Capnp.Rpc.Interception
             switch (cap)
             {
                 case Proxy proxy:
-                    return CapabilityReflection.CreateProxy<TCap>(Attach(policy, proxy.ConsumedCap)) as TCap;
+                    return (CapabilityReflection.CreateProxy<TCap>(Attach(policy, proxy.ConsumedCap!)) as TCap)!;
 
                 case ConsumedCapability ccap:
-                    return new CensorCapability(ccap, policy) as TCap;
+                    return (new CensorCapability(ccap, policy) as TCap)!;
 
                 default:
-                    return Attach(policy, 
+                    return (Attach(policy, 
-                        CapabilityReflection.CreateProxy<TCap>(
+                        (CapabilityReflection.CreateProxy<TCap>(
                             LocalCapability.Create(
-                                Skeleton.GetOrCreateSkeleton(cap, false))) as TCap);
+                                Skeleton.GetOrCreateSkeleton(cap, false))) as TCap)!));
             }
         }
 
@@ -79,11 +80,11 @@ namespace Capnp.Rpc.Interception
             switch (cap)
             {
                 case Proxy proxy:
-                    return CapabilityReflection.CreateProxy<TCap>(Detach(policy, proxy.ConsumedCap)) as TCap;
+                    return (CapabilityReflection.CreateProxy<TCap>(Detach(policy, proxy.ConsumedCap!)) as TCap)!;
 
                 case CensorCapability ccap:
                     {
-                        var cur = ccap;
+                        CensorCapability? cur = ccap;
                         var stk = new Stack<IInterceptionPolicy>();
 
                         do
@@ -96,7 +97,7 @@ namespace Capnp.Rpc.Interception
                                 {
                                     cur2 = p.Attach(cur2);
                                 }
-                                return cur2 as TCap;
+                                return (cur2 as TCap)!;
                             }
 
                             stk.Push(cur.Policy);
@@ -104,7 +105,7 @@ namespace Capnp.Rpc.Interception
                         }
                         while (cur != null);
 
-                        return ccap as TCap;
+                        return (ccap as TCap)!;
                     }
 
                 default:
@@ -113,3 +114,4 @@ namespace Capnp.Rpc.Interception
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/InvalidCapabilityInterfaceException.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// Will be thrown if a type did not qualify as capability interface.
@@ -22,3 +23,4 @@
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/LazyCapability.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class LazyCapability : RefCountingCapability, IResolvingCapability
@@ -27,7 +28,7 @@ namespace Capnp.Rpc
             WhenResolved = capabilityTask;
         }
 
-        internal override void Freeze(out IRpcEndpoint boundEndpoint)
+        internal override void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             if (WhenResolved.IsCompleted)
             {
@@ -37,7 +38,7 @@ namespace Capnp.Rpc
                 }
                 catch (AggregateException exception)
                 {
-                    throw exception.InnerException;
+                    throw exception.InnerException!;
                 }
             }
             else
@@ -106,3 +107,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/LocalAnswer.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class LocalAnswer : IPromisedAnswer
@@ -50,3 +51,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/LocalAnswerCapability.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class LocalAnswerCapability : RefCountingCapability, IResolvingCapability
@@ -15,7 +16,7 @@ namespace Capnp.Rpc
             _access = access;
         }
 
-        internal override void Freeze(out IRpcEndpoint boundEndpoint)
+        internal override void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             boundEndpoint = null;
         }
@@ -43,7 +44,7 @@ namespace Capnp.Rpc
                 }
                 catch (AggregateException exception)
                 {
-                    throw exception.InnerException;
+                    throw exception.InnerException!;
                 }
 
                 using (var proxy = new Proxy(_access.Eval(result)))
@@ -87,3 +88,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/LocalCapability.cs

@@ -4,6 +4,7 @@ using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class LocalCapability : ConsumedCapability
@@ -14,7 +15,7 @@ namespace Capnp.Rpc
         public static ConsumedCapability Create(Skeleton skeleton)
         {
             if (skeleton is Vine vine)
-                return vine.Proxy.ConsumedCap;
+                return vine.Proxy.ConsumedCap!;
             else
                 return _localCaps.GetValue(skeleton, _ => new LocalCapability(_));
         }
@@ -22,7 +23,7 @@ namespace Capnp.Rpc
         static async Task<DeserializerState> AwaitAnswer(Task<AnswerOrCounterquestion> call)
         {
             var aorcq = await call;
-            return aorcq.Answer ?? await aorcq.Counterquestion.WhenReturned;
+            return aorcq.Answer ?? await aorcq.Counterquestion!.WhenReturned;
         }
 
         public Skeleton ProvidedCap { get; }
@@ -55,7 +56,7 @@ namespace Capnp.Rpc
             capDesc.SenderHosted = endpoint.AllocateExport(ProvidedCap, out bool _);
         }
 
-        internal override void Freeze(out IRpcEndpoint boundEndpoint)
+        internal override void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             boundEndpoint = null;
         }
@@ -69,3 +70,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/MemberAccessPath.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -169,7 +170,7 @@ namespace Capnp.Rpc
         /// <param name="rpcState">The object (usually "params struct") on which to evaluate this path.</param>
         /// <returns>Resulting low-level capability</returns>
         /// <exception cref="DeserializationException">Evaluation of this path did not give a capability</exception>
-        public ConsumedCapability Eval(DeserializerState rpcState)
+        public ConsumedCapability? Eval(DeserializerState rpcState)
         {
             var cur = rpcState;
 
@@ -184,7 +185,7 @@ namespace Capnp.Rpc
                     return null;
 
                 case ObjectKind.Capability:
-                    return rpcState.Caps[(int)cur.CapabilityIndex];
+                    return rpcState.Caps![(int)cur.CapabilityIndex];
 
                 default:
                     throw new DeserializationException("Access path did not result in a capability");
@@ -192,3 +193,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/PendingAnswer.cs

@@ -3,19 +3,20 @@ using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class PendingAnswer: IDisposable
     {
         readonly object _reentrancyBlocker = new object();
-        readonly CancellationTokenSource _cts;
+        readonly CancellationTokenSource? _cts;
         readonly TaskCompletionSource<int> _whenCanceled;
         Task<AnswerOrCounterquestion> _callTask;
-        Task _initialTask;
+        Task? _initialTask;
-        Task _chainedTask;
+        Task? _chainedTask;
         bool _disposed;
 
-        public PendingAnswer(Task<AnswerOrCounterquestion> callTask, CancellationTokenSource cts)
+        public PendingAnswer(Task<AnswerOrCounterquestion> callTask, CancellationTokenSource? cts)
         {
             _cts = cts;
             _callTask = callTask ?? throw new ArgumentNullException(nameof(callTask));
@@ -138,7 +139,7 @@ namespace Capnp.Rpc
                             case ObjectKind.Capability:
                                 try
                                 {
-                                    var cap = aorcq.Answer.Caps[(int)cur.CapabilityIndex];
+                                    var cap = aorcq.Answer.Caps![(int)cur.CapabilityIndex];
                                     proxy = new Proxy(cap ?? LazyCapability.Null);
                                 }
                                 catch (ArgumentOutOfRangeException)
@@ -154,7 +155,7 @@ namespace Capnp.Rpc
                     else
                     {
                         var path = MemberAccessPath.Deserialize(rd);
-                        var cap = new RemoteAnswerCapability(aorcq.Counterquestion, path);
+                        var cap = new RemoteAnswerCapability(aorcq.Counterquestion!, path);
                         return new Proxy(cap);
                     }
                 }
@@ -169,7 +170,7 @@ namespace Capnp.Rpc
         {
             if (_cts != null)
             {
-                Task chainedTask;
+                Task? chainedTask;
 
                 lock (_reentrancyBlocker)
                 {
@@ -199,3 +200,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/PendingQuestion.cs

@@ -4,6 +4,7 @@ using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -61,11 +62,11 @@ namespace Capnp.Rpc
 
         readonly TaskCompletionSource<DeserializerState> _tcs = new TaskCompletionSource<DeserializerState>();
         readonly uint _questionId;
-        ConsumedCapability _target;
+        ConsumedCapability? _target;
-        SerializerState _inParams;
+        SerializerState? _inParams;
         int _inhibitFinishCounter;
 
-        internal PendingQuestion(IRpcEndpoint ep, uint id, ConsumedCapability target, SerializerState inParams)
+        internal PendingQuestion(IRpcEndpoint ep, uint id, ConsumedCapability? target, SerializerState? inParams)
         {
             RpcEndpoint = ep ?? throw new ArgumentNullException(nameof(ep));
             _questionId = id;
@@ -75,7 +76,7 @@ namespace Capnp.Rpc
 
             if (inParams != null)
             {
-                foreach (var cap in inParams.Caps)
+                foreach (var cap in inParams.Caps!)
                 {
                     cap?.AddRef();
                 }
@@ -236,7 +237,7 @@ namespace Capnp.Rpc
         /// <param name="access">Access path</param>
         /// <returns>Low-level capability</returns>
         /// <exception cref="DeserializationException">The referenced member does not exist or does not resolve to a capability pointer.</exception>
-        public ConsumedCapability Access(MemberAccessPath access)
+        public ConsumedCapability? Access(MemberAccessPath access)
         {
             lock (ReentrancyBlocker)
             {
@@ -249,7 +250,7 @@ namespace Capnp.Rpc
                     }
                     catch (AggregateException exception)
                     {
-                        throw exception.InnerException;
+                        throw exception.InnerException!;
                     }
                 }
                 else
@@ -259,11 +260,11 @@ namespace Capnp.Rpc
             }
         }
 
-        static void ReleaseCaps(ConsumedCapability target, SerializerState inParams)
+        static void ReleaseCaps(ConsumedCapability? target, SerializerState? inParams)
         {
             if (inParams != null)
             {
-                foreach (var cap in inParams.Caps)
+                foreach (var cap in inParams.Caps!)
                 {
                     cap?.Release();
                 }
@@ -277,7 +278,7 @@ namespace Capnp.Rpc
 
         static void ReleaseOutCaps(DeserializerState outParams)
         {
-            foreach (var cap in outParams.Caps)
+            foreach (var cap in outParams.Caps!)
             {
                 cap?.Release();
             }
@@ -285,12 +286,13 @@ namespace Capnp.Rpc
 
         internal void Send()
         {
-            SerializerState inParams;
+            SerializerState? inParams;
-            ConsumedCapability target;
+            ConsumedCapability? target;
 
             lock (ReentrancyBlocker)
             {
-                Debug.Assert(!StateFlags.HasFlag(State.Sent));
+                if (StateFlags.HasFlag(State.Sent))
+                    throw new InvalidOperationException("Already sent");
 
                 inParams = _inParams;
                 _inParams = null;
@@ -299,7 +301,7 @@ namespace Capnp.Rpc
                 StateFlags |= State.Sent;
             }
 
-            var msg = (Message.WRITER)inParams.MsgBuilder.Root;
+            var msg = (Message.WRITER)inParams!.MsgBuilder!.Root!;
             Debug.Assert(msg.Call.Target.which != MessageTarget.WHICH.undefined);
             var call = msg.Call;
             call.QuestionId = QuestionId;
@@ -316,15 +318,15 @@ namespace Capnp.Rpc
                 OnException(exception);
             }
 
-            ReleaseCaps(target, inParams);
+            ReleaseCaps(target!, inParams);
         }
 
         #region IDisposable Support
 
         void Dispose(bool disposing)
         {
-            SerializerState inParams;
+            SerializerState? inParams;
-            ConsumedCapability target;
+            ConsumedCapability? target;
             bool justDisposed = false;
 
             lock (ReentrancyBlocker)
@@ -377,3 +379,4 @@ namespace Capnp.Rpc
         #endregion
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/PolySkeleton.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -71,3 +72,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/PromisedCapability.cs

@@ -3,6 +3,7 @@ using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class PromisedCapability : RemoteResolvingCapability
@@ -19,7 +20,7 @@ namespace Capnp.Rpc
 
         public override Task<Proxy> WhenResolved => _resolvedCap.Task;
 
-        internal override void Freeze(out IRpcEndpoint boundEndpoint)
+        internal override void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             lock (_reentrancyBlocker)
             {
@@ -31,7 +32,7 @@ namespace Capnp.Rpc
                     }
                     catch (AggregateException exception)
                     {
-                        throw exception.InnerException;
+                        throw exception.InnerException!;
                     }
                 }
                 else
@@ -148,7 +149,7 @@ namespace Capnp.Rpc
             }
         }
 
-        protected override Proxy ResolvedCap
+        protected override Proxy? ResolvedCap
         {
             get
             {
@@ -158,7 +159,7 @@ namespace Capnp.Rpc
                 }
                 catch (AggregateException exception)
                 {
-                    throw exception.InnerException;
+                    throw exception.InnerException!;
                 }
             }
         }
@@ -258,3 +259,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Proxy.cs

@@ -3,6 +3,7 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -38,7 +39,7 @@ namespace Capnp.Rpc
         /// <summary>
         /// Underlying low-level capability
         /// </summary>
-        protected internal ConsumedCapability ConsumedCap { get; private set; }
+        protected internal ConsumedCapability? ConsumedCap { get; private set; }
 
         /// <summary>
         /// Whether is this a broken capability.
@@ -99,12 +100,12 @@ namespace Capnp.Rpc
         {
         }
 
-        internal Proxy(ConsumedCapability cap)
+        internal Proxy(ConsumedCapability? cap)
         {
             Bind(cap);
         }
 
-        internal void Bind(ConsumedCapability cap)
+        internal void Bind(ConsumedCapability? cap)
         {
             if (ConsumedCap != null)
                 throw new InvalidOperationException("Proxy was already bound");
@@ -116,7 +117,7 @@ namespace Capnp.Rpc
             cap.AddRef();
         }
 
-        internal IProvidedCapability GetProvider()
+        internal IProvidedCapability? GetProvider()
         {
             switch (ConsumedCap)
             {
@@ -200,7 +201,7 @@ namespace Capnp.Rpc
 
             using (disposeThis ? this : null)
             {
-                return CapabilityReflection.CreateProxy<T>(ConsumedCap) as T;
+                return (CapabilityReflection.CreateProxy<T>(ConsumedCap) as T)!;
             }
         }
 
@@ -215,7 +216,7 @@ namespace Capnp.Rpc
                 ConsumedCap.Export(endpoint, writer);
         }
 
-        internal void Freeze(out IRpcEndpoint boundEndpoint)
+        internal void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             if (_disposedValue)
                 throw new ObjectDisposedException(nameof(Proxy));
@@ -239,3 +240,4 @@ namespace Capnp.Rpc
 #endif
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/ProxyAttribute.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Text;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -28,3 +29,4 @@ namespace Capnp.Rpc
         public Type ProxyClass { get; }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RefCountingCapability.cs

@@ -2,6 +2,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     abstract class RefCountingCapability: ConsumedCapability
@@ -112,3 +113,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RemoteAnswerCapability.cs

@@ -3,6 +3,7 @@ using System;
 using System.Diagnostics;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class RemoteAnswerCapability : RemoteResolvingCapability
@@ -18,7 +19,7 @@ namespace Capnp.Rpc
 
         readonly PendingQuestion _question;
         readonly MemberAccessPath _access;
-        Proxy _resolvedCap;
+        Proxy? _resolvedCap;
 
         public RemoteAnswerCapability(PendingQuestion question, MemberAccessPath access): base(question.RpcEndpoint)
         {
@@ -59,7 +60,7 @@ namespace Capnp.Rpc
             }
         }
 
-        protected override Proxy ResolvedCap
+        protected override Proxy? ResolvedCap
         {
             get
             {
@@ -74,7 +75,7 @@ namespace Capnp.Rpc
                         }
                         catch (AggregateException exception)
                         {
-                            throw exception.InnerException;
+                            throw exception.InnerException!;
                         }
 
                         _resolvedCap = new Proxy(_access.Eval(result));
@@ -87,7 +88,11 @@ namespace Capnp.Rpc
         async Task<Proxy> AwaitWhenResolved()
         {
             await _question.WhenReturned;
-            return ResolvedCap;
+
+            if (_question.IsTailCall)
+                throw new InvalidOperationException("Question is a tail call, so won't resolve back.");
+
+            return ResolvedCap!;
         }
 
         public override Task<Proxy> WhenResolved => AwaitWhenResolved();
@@ -168,7 +173,7 @@ namespace Capnp.Rpc
             return call;
         }
 
-        internal override void Freeze(out IRpcEndpoint boundEndpoint)
+        internal override void Freeze(out IRpcEndpoint? boundEndpoint)
         {
             lock (_question.ReentrancyBlocker)
             {
@@ -216,7 +221,7 @@ namespace Capnp.Rpc
 
                 if (_question.StateFlags.HasFlag(PendingQuestion.State.Returned))
                 {
-                    ResolvedCap.Export(endpoint, writer);
+                    ResolvedCap?.Export(endpoint, writer);
                 }
                 else
                 {
@@ -255,3 +260,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RemoteCapability.cs

@@ -3,6 +3,7 @@ using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
 
@@ -24,6 +25,9 @@ namespace Capnp.Rpc
 
         protected virtual Call.WRITER SetupMessage(DynamicSerializerState args, ulong interfaceId, ushort methodId)
         {
+            if (args.MsgBuilder == null)
+                throw new ArgumentException("Unbound serializer state", nameof(args));
+
             var callMsg = args.MsgBuilder.BuildRoot<Message.WRITER>();
 
             callMsg.which = Message.WHICH.Call;
@@ -38,3 +42,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RemoteResolvingCapability.cs

@@ -3,6 +3,7 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     abstract class RemoteResolvingCapability : RemoteCapability, IResolvingCapability
@@ -23,14 +24,17 @@ namespace Capnp.Rpc
         }
 
         protected int _pendingCallsOnPromise;
-        Task _disembargo;
+        Task? _disembargo;
 
-        protected abstract Proxy ResolvedCap { get; }
+        protected abstract Proxy? ResolvedCap { get; }
 
         protected abstract void GetMessageTarget(MessageTarget.WRITER wr);
 
         protected IPromisedAnswer CallOnResolution(ulong interfaceId, ushort methodId, DynamicSerializerState args)
         {
+            if (ResolvedCap == null)
+                throw new InvalidOperationException("Capability not yet resolved, calling on resolution not possible");
+
             try
             {
                 ResolvedCap.Freeze(out var resolvedCapEndpoint);
@@ -124,3 +128,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/ResolvingCapabilityExtensions.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     static class ResolvingCapabilityExtensions
     {
@@ -19,7 +20,7 @@
                     {
                         var resolvedCap = await cap.WhenResolved;
 
-                        endpoint.Resolve(preliminaryId, vine, () => resolvedCap.ConsumedCap);
+                        endpoint.Resolve(preliminaryId, vine, () => resolvedCap.ConsumedCap!);
                     }
                     catch (System.Exception exception)
                     {
@@ -42,3 +43,4 @@
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RpcEngine.cs

@@ -10,6 +10,7 @@ using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -65,8 +66,8 @@ namespace Capnp.Rpc
 
         internal class RpcEndpoint : IEndpoint, IRpcEndpoint
         {
-            static readonly ThreadLocal<Action> _exportCapTablePostActions = new ThreadLocal<Action>();
+            static readonly ThreadLocal<Action?> _exportCapTablePostActions = new ThreadLocal<Action?>();
-            static readonly ThreadLocal<PendingQuestion> _tailCall = new ThreadLocal<PendingQuestion>();
+            static readonly ThreadLocal<PendingQuestion?> _tailCall = new ThreadLocal<PendingQuestion?>();
             static readonly ThreadLocal<bool> _canDeferCalls = new ThreadLocal<bool>();
 
             ILogger Logger { get; } = Logging.CreateLogger<RpcEndpoint>();
@@ -145,8 +146,7 @@ namespace Capnp.Rpc
                 Tx(mb.Frame);
             }
 
-            void IRpcEndpoint.Resolve(uint preliminaryId, Skeleton preliminaryCap, 
+            void IRpcEndpoint.Resolve(uint preliminaryId, Skeleton preliminaryCap, Func<ConsumedCapability> resolvedCapGetter)
-                Func<ConsumedCapability> resolvedCapGetter)
             {
                 lock (_reentrancyBlocker)
                 {
@@ -232,7 +232,7 @@ namespace Capnp.Rpc
                 return AllocateExport(providedCapability, out first);
             }
 
-            PendingQuestion AllocateQuestion(ConsumedCapability target, SerializerState inParams)
+            PendingQuestion AllocateQuestion(ConsumedCapability? target, SerializerState? inParams)
             {
                 lock (_reentrancyBlocker)
                 {
@@ -293,7 +293,7 @@ namespace Capnp.Rpc
                 uint q = req.QuestionId;
 
                 var bootstrap = DynamicSerializerState.CreateForRpc();
-                var ans = bootstrap.MsgBuilder.BuildRoot<Message.WRITER>();
+                var ans = bootstrap.MsgBuilder!.BuildRoot<Message.WRITER>();
 
                 ans.which = Message.WHICH.Return;
                 var ret = ans.Return;
@@ -305,7 +305,7 @@ namespace Capnp.Rpc
                 if (bootstrapCap != null)
                 {
                     ret.which = Return.WHICH.Results;
-                    bootstrap.SetCapability(bootstrap.ProvideCapability(LocalCapability.Create(_host.BootstrapCap)));
+                    bootstrap.SetCapability(bootstrap.ProvideCapability(LocalCapability.Create(bootstrapCap)));
                     ret.Results.Content = bootstrap;
 
                     bootstrapTask = Task.FromResult<AnswerOrCounterquestion>(bootstrap);
@@ -371,12 +371,12 @@ namespace Capnp.Rpc
                     }
                     catch (RpcException exception)
                     {
-                        Logger.LogWarning($"Unable to return call: {exception.InnerException.Message}");
+                        Logger.LogWarning($"Unable to return call: {exception.InnerException?.Message ?? exception.Message}");
                     }
                 }
 
-                IProvidedCapability cap;
+                IProvidedCapability? cap;
-                PendingAnswer pendingAnswer = null;
+                PendingAnswer? pendingAnswer = null;
                 bool releaseParamCaps = false;
 
                 void AwaitAnswerAndReply()
@@ -414,8 +414,8 @@ namespace Capnp.Rpc
                                     }
                                     else if (aorcq.Answer != null || aorcq.Counterquestion != _tailCall.Value)
                                     {
-                                        var results = aorcq.Answer ?? (DynamicSerializerState)(await aorcq.Counterquestion.WhenReturned);
+                                        var results = aorcq.Answer ?? (DynamicSerializerState)(await aorcq.Counterquestion!.WhenReturned);
-                                        var ret = SetupReturn(results.MsgBuilder);
+                                        var ret = SetupReturn(results.MsgBuilder!);
 
                                         switch (req.SendResultsTo.which)
                                         {
@@ -569,7 +569,7 @@ namespace Capnp.Rpc
                         case MessageTarget.WHICH.PromisedAnswer:
                             {
                                 bool exists;
-                                PendingAnswer previousAnswer;
+                                PendingAnswer? previousAnswer;
 
                                 lock (_reentrancyBlocker)
                                 {
@@ -578,7 +578,7 @@ namespace Capnp.Rpc
 
                                 if (exists)
                                 {
-                                    previousAnswer.Chain(
+                                    previousAnswer!.Chain(
                                         false,
                                         req.Target.PromisedAnswer,
                                         async t =>
@@ -633,7 +633,7 @@ namespace Capnp.Rpc
 
             void ProcessReturn(Return.READER req)
             {
-                PendingQuestion question;
+                PendingQuestion? question;
 
                 lock (_reentrancyBlocker)
                 {
@@ -674,7 +674,7 @@ namespace Capnp.Rpc
                     case Return.WHICH.TakeFromOtherQuestion:
                         {
                             bool exists;
-                            PendingAnswer pendingAnswer;
+                            PendingAnswer? pendingAnswer;
 
                             lock (_reentrancyBlocker)
                             {
@@ -683,7 +683,7 @@ namespace Capnp.Rpc
 
                             if (exists)
                             {
-                                pendingAnswer.Chain(false, async t =>
+                                pendingAnswer!.Chain(false, async t =>
                                 {
                                     try
                                     {
@@ -747,6 +747,8 @@ namespace Capnp.Rpc
                         lock (_reentrancyBlocker)
                         {
                             var resolvedCap = ImportCap(resolve.Cap);
+                            if (resolvedCap == null)
+                                resolvedCap = LazyCapability.CreateBrokenCap("Failed to resolve this capability");
                             resolvableCap.ResolveTo(resolvedCap);
                         }
                         break;
@@ -935,7 +937,7 @@ namespace Capnp.Rpc
                         {
                             foreach (var cap in results.Caps)
                             {
-                                cap.Release();
+                                cap?.Release();
                             }
                         }
                     });
@@ -984,8 +986,8 @@ namespace Capnp.Rpc
                         try
                         {
                             int icount = checked((int)count);
-                            rc.Release(icount);
+                            rc!.Release(icount);
-                            rc.Cap.Relinquish(icount);
+                            rc!.Cap.Relinquish(icount);
 
                             if (rc.RefCount == 0)
                             {
@@ -1171,7 +1173,7 @@ namespace Capnp.Rpc
                 }
             }
 
-            ConsumedCapability ImportCap(CapDescriptor.READER capDesc)
+            ConsumedCapability? ImportCap(CapDescriptor.READER capDesc)
             {
                 lock (_reentrancyBlocker)
                 {
@@ -1192,8 +1194,7 @@ namespace Capnp.Rpc
                                     rcw.Cap.SetTarget(impCap);
                                 }
 
-                                Debug.Assert(impCap != null);
+                                return impCap!;
-                                return impCap;
                             }
                             else
                             {
@@ -1236,7 +1237,6 @@ namespace Capnp.Rpc
                         case CapDescriptor.WHICH.ReceiverHosted:
                             if (_exportTable.TryGetValue(capDesc.ReceiverHosted, out var rc))
                             {
-                                Debug.Assert(rc.Cap != null);
                                 return LocalCapability.Create(rc.Cap);
                             }
                             else
@@ -1311,9 +1311,9 @@ namespace Capnp.Rpc
                 }
             }
 
-            public IList<ConsumedCapability> ImportCapTable(Payload.READER payload)
+            public IList<ConsumedCapability?> ImportCapTable(Payload.READER payload)
             {
-                var list = new List<ConsumedCapability>();
+                var list = new List<ConsumedCapability?>();
 
                 if (payload.CapTable != null)
                 {
@@ -1341,7 +1341,7 @@ namespace Capnp.Rpc
                 Debug.Assert(_exportCapTablePostActions.Value == null);
                 _exportCapTablePostActions.Value = null;
 
-                payload.CapTable.Init(state.MsgBuilder.Caps.Count);
+                payload.CapTable.Init(state.MsgBuilder!.Caps!.Count);
 
                 int i = 0;
                 foreach (var cap in state.MsgBuilder.Caps)
@@ -1454,7 +1454,7 @@ namespace Capnp.Rpc
                     }
                     catch (RpcException exception)
                     {
-                        Logger.LogWarning($"Unable to release import: {exception.InnerException.Message}");
+                        Logger.LogWarning($"Unable to release import: {exception.InnerException?.Message ?? exception.Message}");
                     }
                 }
             }
@@ -1492,12 +1492,12 @@ namespace Capnp.Rpc
             return inboundEndpoint;
         }
 
-        Skeleton _bootstrapCap;
+        Skeleton? _bootstrapCap;
 
         /// <summary>
         /// Gets or sets the bootstrap capability.
         /// </summary>
-        public Skeleton BootstrapCap
+        public Skeleton? BootstrapCap
         {
             get => _bootstrapCap;
             set
@@ -1509,3 +1509,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RpcException.cs

@@ -1,4 +1,5 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     /// <summary>
     /// Thrown when an RPC-related error condition occurs.
@@ -20,3 +21,4 @@
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/RpcUnimplementedException.cs

@@ -1,6 +1,8 @@
-namespace Capnp.Rpc
+#nullable enable
+namespace Capnp.Rpc
 {
     class RpcUnimplementedException : System.Exception
     {
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Skeleton.cs

@@ -6,6 +6,7 @@ using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -179,8 +180,8 @@ namespace Capnp.Rpc
         ILogger Logger { get; } = Logging.CreateLogger<Skeleton<T>>();
 #endif
 
-        Func<DeserializerState, CancellationToken, Task<AnswerOrCounterquestion>>[] _methods;
+        Func<DeserializerState, CancellationToken, Task<AnswerOrCounterquestion>>[] _methods = null!;
-        CancellationTokenSource _disposed = new CancellationTokenSource();
+        CancellationTokenSource? _disposed = new CancellationTokenSource();
         readonly object _reentrancyBlocker = new object();
         int _pendingCalls;
 
@@ -204,7 +205,7 @@ namespace Capnp.Rpc
         /// <summary>
         /// Gets the underlying capability implementation.
         /// </summary>
-        protected T Impl { get; private set; }
+        protected T Impl { get; private set; } = default!;
 
         /// <summary>
         /// Gets the ID of the implemented interface.
@@ -299,3 +300,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/SkeletonAttribute.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -32,3 +33,4 @@ namespace Capnp.Rpc
         public Type SkeletonClass { get; }
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/TcpRpcClient.cs

@@ -10,6 +10,7 @@ using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -44,17 +45,17 @@ namespace Capnp.Rpc
 
         readonly RpcEngine _rpcEngine;
         readonly TcpClient _client;
-        RpcEngine.RpcEndpoint _inboundEndpoint;
+        RpcEngine.RpcEndpoint? _inboundEndpoint;
-        OutboundTcpEndpoint _outboundEndpoint;
+        OutboundTcpEndpoint? _outboundEndpoint;
-        FramePump _pump;
+        FramePump? _pump;
-        Thread _pumpThread;
+        Thread? _pumpThread;
-        Action _attachTracerAction;
+        Action? _attachTracerAction;
 
         /// <summary>
         /// Gets a Task which completes when TCP is connected. Will be
         /// null until connection is actually requested (either by calling Connect or using appropriate constructor).
         /// </summary>
-        public Task WhenConnected { get; private set; }
+        public Task? WhenConnected { get; private set; }
 
         async Task ConnectAsync(string host, int port)
         {
@@ -152,8 +153,14 @@ namespace Capnp.Rpc
         /// </summary>
         /// <typeparam name="TProxy">Bootstrap capability interface</typeparam>
         /// <returns>A proxy for the bootstrap capability</returns>
+        /// <exception cref="InvalidOperationException">Not connected</exception>
         public TProxy GetMain<TProxy>() where TProxy: class
         {
+            if (WhenConnected == null)
+            {
+                throw new InvalidOperationException("Not connecting");
+            }
+
             if (!WhenConnected.IsCompleted)
             {
                 throw new InvalidOperationException("Connection not yet established");
@@ -164,9 +171,7 @@ namespace Capnp.Rpc
                 throw new InvalidOperationException("Connection not successfully established");
             }
 
-            Debug.Assert(_inboundEndpoint != null);
+            return (CapabilityReflection.CreateProxy<TProxy>(_inboundEndpoint!.QueryMain()) as TProxy)!;
-
-            return CapabilityReflection.CreateProxy<TProxy>(_inboundEndpoint.QueryMain()) as TProxy;
         }
 
         /// <summary>
@@ -178,7 +183,7 @@ namespace Capnp.Rpc
 
             try
             {
-                if (!WhenConnected.Wait(500))
+                if (WhenConnected != null && !WhenConnected.Wait(500))
                 {
                     Logger.LogError("Unable to join connection task within timeout");
                 }
@@ -214,7 +219,7 @@ namespace Capnp.Rpc
 
             _attachTracerAction += () =>
             {
-                _pump.AttachTracer(tracer);
+                _pump?.AttachTracer(tracer);
             };
         }
 
@@ -234,33 +239,34 @@ namespace Capnp.Rpc
         /// <summary>
         /// Gets the number of RPC protocol messages sent by this client so far.
         /// </summary>
-        public long SendCount => _inboundEndpoint.SendCount;
+        public long SendCount => _inboundEndpoint?.SendCount ?? 0;
 
         /// <summary>
         /// Gets the number of RPC protocol messages received by this client so far.
         /// </summary>
-        public long RecvCount => _inboundEndpoint.RecvCount;
+        public long RecvCount => _inboundEndpoint?.RecvCount ?? 0;
 
         /// <summary>
         /// Gets the remote port number which this client is connected to, 
         /// or null if the connection is not yet established.
         /// </summary>
-        public int? RemotePort => ((IPEndPoint)_client.Client?.RemoteEndPoint)?.Port;
+        public int? RemotePort => ((IPEndPoint)_client.Client.RemoteEndPoint)?.Port;
 
         /// <summary>
         /// Gets the local port number which this client using, 
         /// or null if the connection is not yet established.
         /// </summary>
-        public int? LocalPort => ((IPEndPoint)_client.Client?.LocalEndPoint)?.Port;
+        public int? LocalPort => ((IPEndPoint)_client.Client.LocalEndPoint)?.Port;
 
         /// <summary>
         /// Whether the I/O thread is currently running
         /// </summary>
-        public bool IsComputing => _pumpThread.ThreadState == System.Threading.ThreadState.Running;
+        public bool IsComputing => _pumpThread?.ThreadState == System.Threading.ThreadState.Running;
 
         /// <summary>
         /// Whether the I/O thread is waiting for data to receive
         /// </summary>
-        public bool IsWaitingForData => _pump.IsWaitingForData;
+        public bool IsWaitingForData => _pump?.IsWaitingForData ?? false;
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/TcpRpcServer.cs

@@ -6,6 +6,7 @@ using System.Net;
 using System.Net.Sockets;
 using System.Threading;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     /// <summary>
@@ -103,12 +104,12 @@ namespace Capnp.Rpc
             public FramePump Pump { get; private set; }
             public OutboundTcpEndpoint OutboundEp { get; private set; }
             public RpcEngine.RpcEndpoint InboundEp { get; private set; }
-            public Thread PumpRunner { get; private set; }
+            public Thread? PumpRunner { get; private set; }
             public int? LocalPort => ((IPEndPoint)Client.Client.LocalEndPoint)?.Port;
             public int? RemotePort => ((IPEndPoint)Client.Client.RemoteEndPoint)?.Port;
             public long RecvCount => InboundEp.RecvCount;
             public long SendCount => InboundEp.SendCount;
-            public bool IsComputing => PumpRunner.ThreadState == ThreadState.Running;
+            public bool IsComputing => PumpRunner?.ThreadState == ThreadState.Running;
             public bool IsWaitingForData => Pump.IsWaitingForData;
 
             public void AttachTracer(IFrameTracer tracer)
@@ -165,7 +166,7 @@ namespace Capnp.Rpc
                         connection.Start();
                     }
 
-                    connection.PumpRunner.Start();
+                    connection.PumpRunner!.Start();
                 }
             }
             catch (SocketException)
@@ -180,8 +181,13 @@ namespace Capnp.Rpc
             }
         }
 
-        void SafeJoin(Thread thread)
+        void SafeJoin(Thread? thread)
         {
+            if (thread == null)
+            {
+                return;
+            }
+
             for (int retry = 0; retry < 5; ++retry)
             {
                 try
@@ -313,6 +319,7 @@ namespace Capnp.Rpc
         /// <summary>
         /// Fires when a new incoming connection was accepted, or when an active connection is closed.
         /// </summary>
-        public event Action<object, ConnectionEventArgs> OnConnectionChanged;
+        public event Action<object, ConnectionEventArgs>? OnConnectionChanged;
     }
 }
+#nullable restore

Capnp.Net.Runtime/Rpc/Vine.cs

@@ -4,6 +4,7 @@ using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 
+#nullable enable
 namespace Capnp.Rpc
 {
     class Vine : Skeleton
@@ -70,3 +71,4 @@ namespace Capnp.Rpc
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/SecurityOptions.cs

@@ -1,4 +1,5 @@
-namespace Capnp
+#nullable enable
+namespace Capnp
 {
     /// <summary>
     /// Provides the security bounds for defeating amplification and stack overflow DoS attacks.
@@ -16,3 +17,4 @@
         public static int RecursionLimit { get; set; } = 64;
     }
 }
+#nullable restore

Capnp.Net.Runtime/SegmentAllocator.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -145,3 +146,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/SegmentSlice.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -18,3 +19,4 @@ namespace Capnp
         public int Offset;
     }
 }
+#nullable restore

Capnp.Net.Runtime/SerializerExtensions.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Runtime.InteropServices;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -339,3 +340,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/SerializerState.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Runtime.InteropServices;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -29,10 +30,10 @@ namespace Capnp
             return s;
         }
 
-        internal MessageBuilder MsgBuilder { get; set; }
+        internal MessageBuilder? MsgBuilder { get; set; }
-        internal ISegmentAllocator Allocator => MsgBuilder?.Allocator;
+        internal ISegmentAllocator? Allocator => MsgBuilder?.Allocator;
-        internal List<Rpc.ConsumedCapability> Caps => MsgBuilder?.Caps;
+        internal List<Rpc.ConsumedCapability?>? Caps => MsgBuilder?.Caps;
-        internal SerializerState Owner { get; set; }
+        internal SerializerState? Owner { get; set; }
         internal int OwnerSlot { get; set; }
         internal uint SegmentIndex { get; set; }
         internal int Offset { get; set; }
@@ -43,7 +44,7 @@ namespace Capnp
         internal ObjectKind Kind { get; set; }
         internal uint CapabilityIndex { get; set; }
 
-        SerializerState[] _linkedStates;
+        SerializerState[]? _linkedStates;
 
         /// <summary>
         /// Constructs an unbound serializer state.
@@ -129,7 +130,7 @@ namespace Capnp
             if (Owner != null)
                 ts.Bind(Owner, OwnerSlot);
             else
-                ts.Bind(MsgBuilder);
+                ts.Bind(MsgBuilder ?? throw Unbound());
 
             return ts;
         }
@@ -152,8 +153,8 @@ namespace Capnp
         /// </summary>
         public bool IsAllocated => Offset >= 0;
 
-        Span<ulong> SegmentSpan =>  IsAllocated ? Allocator.Segments[(int)SegmentIndex].Span : Span<ulong>.Empty;
+        Span<ulong> SegmentSpan => IsAllocated && Allocator != null ? Allocator.Segments[(int)SegmentIndex].Span : Span<ulong>.Empty;
-        Span<ulong> FarSpan(uint index) => Allocator.Segments[(int)index].Span;
+        Span<ulong> FarSpan(uint index) => Allocator!.Segments[(int)index].Span;
 
         /// <summary>
         /// Given this state describes a struct and is allocated, returns the struct's data section.
@@ -181,6 +182,9 @@ namespace Capnp
             }
             else
             {
+                if (Allocator == null)
+                    throw Unbound();
+
                 SegmentIndex = Owner?.SegmentIndex ?? SegmentIndex;
                 Allocator.Allocate(count, SegmentIndex, out var slice, false);
                 SegmentIndex = slice.SegmentIndex;
@@ -191,7 +195,7 @@ namespace Capnp
         }
 
         /// <summary>
-        /// Allocates storage for the underlying object. Does nothing if it is already allocated. From the point the object is allocated, its type cannot by changed
+        /// Allocates storage for the underlying object. Does nothing if it is already allocated. From the point the object is allocated, its type cannot be changed
         /// anymore (e.g. changing from struct to list, or modifying the struct's section sizes).
         /// </summary>
         public void Allocate()
@@ -250,6 +254,9 @@ namespace Capnp
             if (!target.IsAllocated)
                 throw new InvalidOperationException("Target must be allocated before a pointer can be built");
 
+            if (MsgBuilder == null)
+                throw Unbound();
+
             try
             {
                 if (SegmentSpan[offset] != 0)
@@ -345,7 +352,7 @@ namespace Capnp
             {
                 WirePointer farPtr = default;
 
-                if (Allocator.Allocate(1, target.SegmentIndex, out var landingPadSlice, true))
+                if (Allocator!.Allocate(1, target.SegmentIndex, out var landingPadSlice, true))
                 {
                     farPtr.SetFarPointer(target.SegmentIndex, landingPadSlice.Offset, false);
                     SegmentSpan[offset] = farPtr;
@@ -372,7 +379,7 @@ namespace Capnp
             }
         }
 
-        internal Rpc.ConsumedCapability DecodeCapPointer(int offset)
+        internal Rpc.ConsumedCapability? DecodeCapPointer(int offset)
         {
             if (offset < 0)
                 throw new IndexOutOfRangeException(nameof(offset));
@@ -455,7 +462,7 @@ namespace Capnp
                     throw new InvalidOperationException("This object cannot own pointers to sub-objects");
             }
 
-            _linkedStates[slot] = target;
+            _linkedStates![slot] = target;
         }
 
         /// <summary>
@@ -476,6 +483,7 @@ namespace Capnp
         }
 
         static InvalidOperationException AlreadySet() => new InvalidOperationException("The object type was already set");
+        static InvalidOperationException Unbound() => new InvalidOperationException("This state is not bound to a MessageBuilder");
 
         void VerifyNotYetAllocated()
         {
@@ -654,14 +662,21 @@ namespace Capnp
         /// <exception cref="EncoderFallbackException">Trying to obtain the UTF-8 encoding might throw this exception.</exception>
         /// <exception cref="InvalidOperationException">The object type was already set to something different</exception>
         /// <exception cref="ArgumentOutOfRangeException">UTF-8 encoding exceeds 2^29-2 bytes</exception>
-        protected void WriteText(string text)
+        protected void WriteText(string? text)
         {
-            byte[] srcBytes = Encoding.UTF8.GetBytes(text);
+            if (text == null)
-            SetListOfValues(8, srcBytes.Length + 1);
+            {
-            var srcSpan = new ReadOnlySpan<byte>(srcBytes);
+                VerifyNotYetAllocated();
-            var dstSpan = ListGetBytes();
+            }
-            dstSpan = dstSpan.Slice(0, dstSpan.Length - 1);
+            else
-            srcSpan.CopyTo(dstSpan);
+            {
+                byte[] srcBytes = Encoding.UTF8.GetBytes(text);
+                SetListOfValues(8, srcBytes.Length + 1);
+                var srcSpan = new ReadOnlySpan<byte>(srcBytes);
+                var dstSpan = ListGetBytes();
+                dstSpan = dstSpan.Slice(0, dstSpan.Length - 1);
+                srcSpan.CopyTo(dstSpan);
+            }
         }
 
         /// <summary>
@@ -761,7 +776,7 @@ namespace Capnp
             if (Kind != ObjectKind.Struct && Kind != ObjectKind.ListOfPointers)
                 throw new InvalidOperationException("This is not a struct or list of pointers");
 
-            ref var state = ref _linkedStates[index];
+            ref var state = ref _linkedStates![index];
 
             if (state == null)
             {
@@ -784,12 +799,12 @@ namespace Capnp
         /// <item><description>Object at given position is not compatible with the desired target serializer type.</description></item>
         /// </list></exception>
         /// <exception cref="IndexOutOfRangeException"><paramref name="index"/> is out of bounds.</exception>
-        public TS TryGetPointer<TS>(int index) where TS : SerializerState, new()
+        public TS? TryGetPointer<TS>(int index) where TS : SerializerState, new()
         {
             if (Kind != ObjectKind.Struct && Kind != ObjectKind.ListOfPointers)
                 throw new InvalidOperationException("This is not a struct or list of pointers");
 
-            var state = _linkedStates[index];
+            var state = _linkedStates![index];
 
             if (state == null) return null;
 
@@ -820,7 +835,7 @@ namespace Capnp
         /// <item><description>Object at given position is not compatible with the desired target serializer type.</description></item>
         /// </list></exception>
         /// <exception cref="IndexOutOfRangeException"><paramref name="index"/> is out of bounds.</exception>
-        public SerializerState TryGetPointer(int index) => TryGetPointer<SerializerState>(index);
+        public SerializerState? TryGetPointer(int index) => TryGetPointer<SerializerState>(index);
 
         /// <summary>
         /// Reads text from a struct field or list element.
@@ -829,7 +844,7 @@ namespace Capnp
         /// If the underlying object is a list of pointers: Element index</param>
         /// <param name="defaultText">String to return in case of null</param>
         /// <returns>The decoded text</returns>
-        public string ReadText(int index, string defaultText = null)
+        public string? ReadText(int index, string? defaultText = null)
         {
             var b = BuildPointer(index);
 
@@ -851,7 +866,7 @@ namespace Capnp
         /// <item><description>Object at given position was already set.</description></item>
         /// </list></exception>
         /// <exception cref="IndexOutOfRangeException"><paramref name="index"/> is out of bounds.</exception>
-        public void WriteText(int index, string text)
+        public void WriteText(int index, string? text)
         {
             BuildPointer(index).WriteText(text);
         }
@@ -886,11 +901,11 @@ namespace Capnp
             if (Kind != ObjectKind.ListOfStructs)
                 throw new InvalidOperationException("This is not a list of structs");
 
-            ref var state = ref _linkedStates[index];
+            ref var state = ref _linkedStates![index];
 
             if (state == null)
             {
-                state = new SerializerState(MsgBuilder);
+                state = new SerializerState(MsgBuilder!);
                 state.SetStruct(StructDataCount, StructPtrCount);
                 state.SegmentIndex = SegmentIndex;
                 state.Offset = Offset + 1 + index * (StructDataCount + StructPtrCount);
@@ -913,12 +928,12 @@ namespace Capnp
             if (Kind != ObjectKind.ListOfStructs)
                 throw new InvalidOperationException("This is not a list of structs");
 
-            ref var state = ref _linkedStates[index];
+            ref var state = ref _linkedStates![index];
 
             if (state == null)
             {
                 state = new TS();
-                state.Bind(MsgBuilder);
+                state.Bind(MsgBuilder!);
                 state.SetStruct(StructDataCount, StructPtrCount);
                 state.SegmentIndex = SegmentIndex;
                 int stride = StructDataCount + StructPtrCount;
@@ -942,19 +957,19 @@ namespace Capnp
 
             for (int offset = minOffset; offset < maxOffset; offset++)
             {
-                ref var state = ref _linkedStates[offset - minOffset];
+                ref var state = ref _linkedStates![offset - minOffset];
 
                 if (state == null)
                 {
                     state = new TS();
-                    state.Bind(MsgBuilder);
+                    state.Bind(MsgBuilder!);
                     state.SetStruct(StructDataCount, StructPtrCount);
                     state.SegmentIndex = SegmentIndex;
                     state.Offset = offset;
                 }
             }
 
-            return _linkedStates.LazyListSelect(ts => (TS)ts);
+            return _linkedStates!.LazyListSelect(ts => (TS)ts);
         }
 
         /// <summary>
@@ -1240,7 +1255,7 @@ namespace Capnp
         /// <param name="capability">The low-level capability object to provide.</param>
         /// <returns>Index of the given capability in the capability table</returns>
         /// <exception cref="InvalidOperationException">The underlying message builder was not configured for capability table support.</exception>
-        public uint ProvideCapability(Rpc.ConsumedCapability capability)
+        public uint ProvideCapability(Rpc.ConsumedCapability? capability)
         {
             if (Caps == null)
                 throw new InvalidOperationException("Underlying MessageBuilder was not enabled to support capabilities");
@@ -1279,7 +1294,7 @@ namespace Capnp
         /// </list></param>
         /// <returns>Index of the given capability in the capability table</returns>
         /// <exception cref="InvalidOperationException">The underlying message builder was not configured for capability table support.</exception>
-        public uint ProvideCapability(object obj)
+        public uint ProvideCapability(object? obj)
         {
             if (obj == null)
                 return ProvideCapability(default(Rpc.ConsumedCapability));
@@ -1352,7 +1367,7 @@ namespace Capnp
             }
         }
 
-        internal Rpc.ConsumedCapability StructReadRawCap(int index)
+        internal Rpc.ConsumedCapability? StructReadRawCap(int index)
         {
             if (Kind != ObjectKind.Struct && Kind != ObjectKind.Nil)
                 throw new InvalidOperationException("Allowed on structs only");
@@ -1372,7 +1387,7 @@ namespace Capnp
         /// <exception cref="ArgumentOutOfRangeException"><paramref name="slot"/> is out of range.</exception>
         /// <exception cref="ArgumentException">The desired interface does not qualify as capability interface (<see cref="Rpc.ProxyAttribute"/>)</exception>
         /// <exception cref="InvalidOperationException">This state does not represent a struct.</exception>
-        public T ReadCap<T>(int slot) where T : class
+        public T? ReadCap<T>(int slot) where T : class
         {
             var cap = StructReadRawCap(slot);
             return Rpc.CapabilityReflection.CreateProxy<T>(cap) as T;
@@ -1392,3 +1407,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/TypeIdAttribute.cs

@@ -1,5 +1,6 @@
 using System;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -25,3 +26,4 @@ namespace Capnp
         public ulong Id { get; }
     }
 }
+#nullable restore

Capnp.Net.Runtime/UtilityExtensions.cs

@@ -4,8 +4,9 @@ using System.Threading.Tasks;
 
 namespace Capnp
 {
-  internal static class UtilityExtensions
+#nullable enable
-  {
+    internal static class UtilityExtensions
+    {
         /// <summary>
         /// This method exists until NET Standard 2.1 is released
         /// </summary>
@@ -23,7 +24,7 @@ namespace Capnp
             return true;
         }
 #else
-        public static bool ReplacementTryAdd<K, V>(this Dictionary<K, V> thisDict, K key, V value) => thisDict.TryAdd(key, value);
+        public static bool ReplacementTryAdd<K, V>(this Dictionary<K, V> thisDict, K key, V value) where K: struct => thisDict.TryAdd(key, value);
 #endif
 
         /// <summary>
@@ -40,14 +41,16 @@ namespace Capnp
         {
             if (!thisDict.ContainsKey(key))
             {
-                value = default;
+                value = default!; // OK here since .NET Standard 2.0 does not support (non-)nullability anyway.
                 return false;
             }
             value = thisDict[key];
             return thisDict.Remove(key);
         }
 #else
+#pragma warning disable CS8714
         public static bool ReplacementTryRemove<K, V>(this Dictionary<K, V> thisDict, K key, out V value) => thisDict.Remove(key, out value);
+#pragma warning restore CS8714
 #endif
 
         /// <summary>
@@ -77,3 +80,4 @@ namespace Capnp
 #endif
     }
 }
+#nullable restore

Capnp.Net.Runtime/WireFrame.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -22,3 +23,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore

Capnp.Net.Runtime/WirePointer.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Text;
 
+#nullable enable
 namespace Capnp
 {
     /// <summary>
@@ -228,3 +229,4 @@ namespace Capnp
         }
     }
 }
+#nullable restore