Fix sync hang issue after async task method call (#5634)

src/System.Private.ServiceModel/tests/Common/Scenarios/ServiceInterfaces.cs

@@ -89,6 +89,9 @@ public interface IWcfService
 
     [OperationContract]
     Dictionary<string, string> GetRequestHttpHeaders();
+
+    [OperationContract]
+    Task EchoReturnTaskAsync();
 }
 
 [ServiceContract]

src/System.Private.ServiceModel/tests/Scenarios/Binding/Tcp/NetTcpBindingTests.4.0.0.cs

@@ -82,6 +82,45 @@ public static async Task SecurityModeNone_Echo_RoundTrips_String_SyncAfterAsync(
         }
     }
 
+    // Test for https://github.com/dotnet/wcf/issues/5626
+    [WcfFact]
+    [OuterLoop]
+    public static async Task SecurityModeNone_Echo_RoundTrips_Task_SyncAfterAsync()
+    {
+        string testString = "Hello";
+        ChannelFactory<IWcfService> factory = null;
+        IWcfService serviceProxy = null;
+
+        try
+        {
+            // *** SETUP *** \\
+            NetTcpBinding binding = new NetTcpBinding(SecurityMode.None);
+            factory = new ChannelFactory<IWcfService>(binding, new EndpointAddress(Endpoints.Tcp_NoSecurity_Address));
+            serviceProxy = factory.CreateChannel();
+
+            // *** EXECUTE *** \\
+            string result = serviceProxy.Echo(testString);
+            Assert.Equal(testString, result);
+
+            // Without the ConfigureAwait, the xunit sync context will hop threads from the
+            // completing thread to their sync context. ConfigureAwait means it continues
+            // executing on the completing thread and triggers the bug (without the fix).
+            await serviceProxy.EchoReturnTaskAsync().ConfigureAwait(false);
+
+            result = serviceProxy.Echo(testString);
+            Assert.Equal(testString, result);
+
+            // *** CLEANUP *** \\
+            ((ICommunicationObject)serviceProxy).Close();
+            factory.Close();
+        }
+        finally
+        {
+            // *** ENSURE CLEANUP *** \\
+            ScenarioTestHelpers.CloseCommunicationObjects((ICommunicationObject)serviceProxy, factory);
+        }
+    }
+
     // Test for https://github.com/dotnet/wcf/issues/5134
     [WcfFact]
     [OuterLoop]

src/System.Private.ServiceModel/tools/IISHostedWcfService/App_code/IWcfService.cs

@@ -160,5 +160,8 @@ public interface IWcfService
 
         [OperationContract]
         Dictionary<string,string> GetRequestHttpHeaders();
+
+        [OperationContract]
+        System.Threading.Tasks.Task EchoReturnTask();
     }
 }

src/System.Private.ServiceModel/tools/IISHostedWcfService/App_code/WcfService.cs

@@ -468,5 +468,13 @@ private static Stream StringToStream(string str)
 
             return ms;
         }
+
+        public System.Threading.Tasks.Task EchoReturnTask()
+        {
+            return System.Threading.Tasks.Task.Run(() =>
+            {
+                System.Threading.Tasks.Task.Delay(1).Wait();                
+            });
+        }
     }
 }

src/System.ServiceModel.Primitives/src/System/ServiceModel/Channels/ServiceChannelProxy.cs

@@ -210,7 +210,7 @@ private static Task CreateGenericTask(ServiceChannel channel, ProxyOperationRunt
 
             private static Task CreateTask(ServiceChannel channel, ProxyOperationRuntime operation, object[] inputParameters)
             {
-                TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
+                TaskCompletionSource<object> tcs = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);
                 bool completedCallback = false;
 
                 Action<IAsyncResult> endCallDelegate = (asyncResult) =>