Fix C# connection to correctly mark message as sent (#3072)

csharp/src/Ice/ConnectionI.cs

@@ -607,15 +607,23 @@ public override bool startAsync(int operation, Ice.Internal.AsyncCallback comple
                             observerStartWrite(_writeStream.getBuffer());
                         }
 
-                        bool completed;
-                        if (_transceiver.startWrite(_writeStream.getBuffer(), completedCallback, this, out completed))
+                        bool messageWritten = false;
+                        bool completedSynchronously =
+                            _transceiver.startWrite(
+                                _writeStream.getBuffer(),
+                                completedCallback,
+                                this,
+                                out messageWritten);
+                        // If the startWrite call consumed the complete buffer, we assume the message is sent now for
+                        // at-most-once semantics.
+                        if (messageWritten && _sendStreams.Count > 0)
                         {
-                            // If the write completed immediately and the buffer
-                            if (completed && _sendStreams.Count > 0)
-                            {
-                                // The whole message is written, assume it's sent now for at-most-once semantics.
-                                _sendStreams.First.Value.isSent = true;
-                            }
+                            _sendStreams.First.Value.isSent = true;
+                        }
+
+                        if (completedSynchronously)
+                        {
+                            // If the write completed synchronously, we need to call the completedCallback.
                             completedCallback(this);
                         }
                     }