Add generic variance to reader and writer interfaces

src/csharp/Grpc.Core.Api/IAsyncStreamReader.cs

@@ -47,7 +47,7 @@ namespace Grpc.Core
     /// </para>
     /// </summary>
     /// <typeparam name="T">The message type.</typeparam>
-    public interface IAsyncStreamReader<T>
+    public interface IAsyncStreamReader<out T>
     {
         /// <summary>
         /// Gets the current element in the iteration.

src/csharp/Grpc.Core.Api/IAsyncStreamWriter.cs

@@ -1,4 +1,4 @@
-#region Copyright notice and license
+#region Copyright notice and license
 
 // Copyright 2015 gRPC authors.
 //
@@ -28,7 +28,7 @@ namespace Grpc.Core
     /// A writable stream of messages.
     /// </summary>
     /// <typeparam name="T">The message type.</typeparam>
-    public interface IAsyncStreamWriter<T>
+    public interface IAsyncStreamWriter<in T>
     {
         /// <summary>
         /// Writes a single asynchronously. Only one write can be pending at a time.

src/csharp/Grpc.Core.Api/IClientStreamWriter.cs

@@ -1,4 +1,4 @@
-#region Copyright notice and license
+#region Copyright notice and license
 
 // Copyright 2015 gRPC authors.
 //
@@ -28,7 +28,7 @@ namespace Grpc.Core
     /// Client-side writable stream of messages with Close capability.
     /// </summary>
     /// <typeparam name="T">The message type.</typeparam>
-    public interface IClientStreamWriter<T> : IAsyncStreamWriter<T>
+    public interface IClientStreamWriter<in T> : IAsyncStreamWriter<T>
     {
         /// <summary>
         /// Completes/closes the stream. Can only be called once there is no pending write. No writes should follow calling this.

src/csharp/Grpc.Core.Api/IServerStreamWriter.cs

@@ -1,4 +1,4 @@
-#region Copyright notice and license
+#region Copyright notice and license
 
 // Copyright 2015 gRPC authors.
 //
@@ -27,7 +27,7 @@ namespace Grpc.Core
     /// <summary>
     /// A writable stream of messages that is used in server-side handlers.
     /// </summary>
-    public interface IServerStreamWriter<T> : IAsyncStreamWriter<T>
+    public interface IServerStreamWriter<in T> : IAsyncStreamWriter<T>
     {
         // TODO(jtattermusch): consider just using IAsyncStreamWriter instead of this interface.
     }