exception:在已经完成 任务后，尝试将任务转换为最终状态。

refs:

https://stackoverflow.com/questions/15316613/when-should-taskcompletionsourcet-be-used

https://technet.microsoft.com/zh-cn/library/dd449174(v=vs.110).aspx

https://code.msdn.microsoft.com/Windows-Phone-8-Networking-835239c1/sourcecode?fileId=70769&pathId=221052408

相关

https://hmemcpy.com/2013/01/turning-old-and-busted-asynchronous-code-into-new-asyncawait-enabled-hotness-with-taskcompletionsourcet/

An attempt was made to transition a task to a final state when it had already completed

TaskCompletionSource is used to create Task objects that don't execute code. 

In Real World Scenarios TaskCompletionSource is ideal for I/O bound operations. 

This way you get all the benefits of tasks (e.g. return values, continuations etc) without blocking a thread for the duration of the operation. 

If your "function" is an IO bound operation it isn't recommended to block a thread using a new Task. 

Instead using TaskCompletionSource you can create a slave task to just indicate when your I/O bound operation finishes or faults.

TaskCompletionSource 对于非task型方法，并且有事务需要处理，可返回task，并带上result或exception
如(only a event based api is available )

 TaskCompletionSource<object> completionSource;         public Task SendUsingManagedSocketsAsync(string strServerIP)         {             // enable asynchronous task completion             completionSource = new TaskCompletionSource<object>();             // create a new socket              var socket = new Socket(AddressFamily.InterNetwork,                 SocketType.Stream,                 ProtocolType.Tcp);              // create endpoint              var ipAddress = IPAddress.Parse(strServerIP);             var endpoint = new IPEndPoint(ipAddress, PORT);              // create event args              var args = new SocketAsyncEventArgs();             args.RemoteEndPoint = endpoint;             args.Completed += SocketConnectCompleted;              // check if the completed event will be raised. If not, invoke the handler manually.              if (!socket.ConnectAsync(args))                 SocketConnectCompleted(args.ConnectSocket, args);              return completionSource.Task;         }          private void SocketConnectCompleted(object sender, SocketAsyncEventArgs e)         {             // check for errors              if (e.SocketError != System.Net.Sockets.SocketError.Success)             {                 completionSource.SetException(new Exception("Failed with " + e.SocketError));                  // do some resource cleanup                  CleanUp(e);                 return;             } else             {                 completionSource.SetResult(null);             }              // check what has been executed              switch (e.LastOperation)             {                 case SocketAsyncOperation.Connect:                     HandleConnect(e);                     break;                 case SocketAsyncOperation.Send:                     HandleSend(e);                     break;             }         }