add concept of CallInvoker
diff --git a/src/csharp/Grpc.Core/CallInvoker.cs b/src/csharp/Grpc.Core/CallInvoker.cs
new file mode 100644
index 0000000..e8e4396
--- /dev/null
+++ b/src/csharp/Grpc.Core/CallInvoker.cs
@@ -0,0 +1,84 @@
+#region Copyright notice and license
+
+// Copyright 2015, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+// * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+// * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+// * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#endregion
+
+using System.Threading.Tasks;
+using Grpc.Core.Internal;
+
+namespace Grpc.Core
+{
+ /// <summary>
+ /// Abstraction of client-side RPC invocation.
+ /// </summary>
+ /// <seealso cref="Calls"/>
+ public abstract class CallInvoker
+ {
+ /// <summary>
+ /// Invokes a simple remote call in a blocking fashion.
+ /// </summary>
+ public abstract TResponse BlockingUnaryCall<TRequest, TResponse>(Method<TRequest, TResponse> method, CallOptions options, TRequest request)
+ where TRequest : class
+ where TResponse : class;
+
+ /// <summary>
+ /// Invokes a simple remote call asynchronously.
+ /// </summary>
+ public abstract AsyncUnaryCall<TResponse> AsyncUnaryCall<TRequest, TResponse>(Method<TRequest, TResponse> method, CallOptions options, TRequest request)
+ where TRequest : class
+ where TResponse : class;
+
+ /// <summary>
+ /// Invokes a server streaming call asynchronously.
+ /// In server streaming scenario, client sends on request and server responds with a stream of responses.
+ /// </summary>
+ public abstract AsyncServerStreamingCall<TResponse> AsyncServerStreamingCall<TRequest, TResponse>(Method<TRequest, TResponse> method, CallOptions options, TRequest request)
+ where TRequest : class
+ where TResponse : class;
+
+ /// <summary>
+ /// Invokes a client streaming call asynchronously.
+ /// In client streaming scenario, client sends a stream of requests and server responds with a single response.
+ /// </summary>
+ public abstract AsyncClientStreamingCall<TRequest, TResponse> AsyncClientStreamingCall<TRequest, TResponse>(Method<TRequest, TResponse> method, CallOptions options)
+ where TRequest : class
+ where TResponse : class;
+
+ /// <summary>
+ /// Invokes a duplex streaming call asynchronously.
+ /// In duplex streaming scenario, client sends a stream of requests and server responds with a stream of responses.
+ /// The response stream is completely independent and both side can be sending messages at the same time.
+ /// </summary>
+ public abstract AsyncDuplexStreamingCall<TRequest, TResponse> AsyncDuplexStreamingCall<TRequest, TResponse>(Method<TRequest, TResponse> method, CallOptions options)
+ where TRequest : class
+ where TResponse : class;
+ }
+}