Add .NET 6 target

This doesn't unlock any scenarios, but it does allow dropping of the explicit `System.Runtime.CompilerServices.Unsafe` dependency for such programs.

Author: AArnott

src/Nerdbank.Streams/FullDuplexStream.cs

@@ -143,10 +143,10 @@ public override Task WriteAsync(byte[] buffer, int offset, int count, Cancellati
 
             public override void WriteByte(byte value) => this.writableStream.WriteByte(value);
 
-            public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+            public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback? callback, object? state)
                 => this.readableStream.BeginRead(buffer, offset, count, callback, state);
 
-            public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+            public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback? callback, object? state)
                 => this.writableStream.BeginWrite(buffer, offset, count, callback, state);
 
             public override int EndRead(IAsyncResult asyncResult) => this.readableStream.EndRead(asyncResult);

src/Nerdbank.Streams/MultiplexingStream.Channel.cs

@@ -368,7 +368,7 @@ internal void Dispose(Exception? disposeException)
                         {
                             try
                             {
-                                Channel self = (Channel)state;
+                                Channel self = (Channel)state!;
 
                                 PipeWriter? mxStreamIOWriter;
                                 lock (self.SyncObject)

src/Nerdbank.Streams/Nerdbank.Streams.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>net6.0;netstandard2.0;netstandard2.1</TargetFrameworks>
     <DefineConstants>$(DefineConstants);MESSAGEPACK_INTERNAL;CSHARP8</DefineConstants>
 
     <Summary>Streams for full duplex in-proc communication, wrap a WebSocket, split a stream into multiple channels, etc.</Summary>
@@ -15,7 +15,7 @@
     <PackageReference Include="Microsoft.VisualStudio.Threading.Analyzers" PrivateAssets="all" />
     <PackageReference Include="Microsoft.VisualStudio.Validation" />
     <PackageReference Include="System.IO.Pipelines" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Condition="'$(TargetFramework)' != 'net6.0'" />
   </ItemGroup>
   <ItemGroup>
     <Compile Update="Strings.Designer.cs">

src/Nerdbank.Streams/Substream.cs

@@ -64,7 +64,7 @@ public override long Position
 
         /// <inheritdoc/>
         public
-#if NETSTANDARD2_1_OR_GREATER
+#if NETSTANDARD2_1_OR_GREATER || NET6_0_OR_GREATER
 new // https://github.com/dotnet/csharplang/issues/3613: There's no way to *override* the base method without a binary breaking change from changing the return type on this method.
 #endif
         Task DisposeAsync() => this.DisposeAsync(CancellationToken.None).AsTask();