C# 在基于事件的异步模式上使用任务并行库

我正在写一个网络应用程序

消息通过以下传输方式发送：

Network.SendMessage (new FirstMessage() );

我可以注册一个事件处理程序，在该消息类型到达时调用，如下所示：

Network.RegisterMessageHandler<FirstMessage> (OnFirstMessageReceived);

public void OnFirstMessageReceived(EventArgs<FirstMessageEventArgs> e)
{
     Network.SendMessage( new SecondMessage() );
}

public void OnSecondMessageReceived(EventArgs<SecondMessageEventArgs> e)
{
     Network.SendMessage( new ThirdMessage() );
}

public void OnThirdMessageReceived(EventArgs<ThirdMessageEventArgs> e)
{
     Network.SendMessage( new FourthMessage() );
}

public void OnFourthMessageReceived(EventArgs<FourthMessageEventArgs> e)
{
     // Authentication is complete
}

public void Drink() {}
public void Eat()   {}
public void Sleep() {}

Task.Factory.StartNew(     () => Drink() )
            .ContinueWith( () => Eat()   )
            .ContinueWith( () => Sleep() );

这与我的基于事件的异步模式相反，在这种模式中，只有在接收到消息时才调用每个事件处理程序方法

换句话说，我不能做这样的事情（但我想）：

---

我读过，但不太明白。似乎我需要的是

TaskCompletionSource

。如果我想从基于事件的异步模式（如上面的代码块）创建一个任务，它会是什么样子？

Jeremy Likness有一个博客标题，您可能会感兴趣。以下是他试图回答的问题：

这个概念很简单：我们经常希望一组异步操作按顺序执行。也许您必须从服务加载列表，然后加载所选项目，然后触发动画。这可以通过链接已完成的事件或嵌套lambda表达式来实现，但是有更干净的方法吗

---

关于TaskCompletionSource，您是对的，它是将EAP（基于事件的异步模式）转换为TPL任务的关键

这里记录了这一点：

以下是简化代码：

public static class Extensions  
{
    public static Task<XDocument> GetRssDownloadTask(
        this WebClient client, Uri rssFeedUri)
    {
        // task completion source is an object, which has some state.
        // it gives out the task, which completes, when state turns "completed"
        // or else it could be canceled or throw an exception
        var tcs = new TaskCompletionSource<XDocument>(); 

        // now we subscribe to completed event. depending on event result
        // we set TaskCompletionSource state completed, canceled, or error
        client.DownloadStringCompleted += (sender, e) => 
        {
                  if(e.Cancelled) 
                  {
                      tcs.SetCanceled();
                  }
                  else if(null != e.Error)
                  {
                      tcs.SetException(e.Error);
                  }
                  else
                  { 
                      tcs.SetResult(XDocument.Parse(e.Result));
                  }
        };

        // now we start asyncronous operation
        client.DownloadStringAsync(rssFeedUri);
        // and return the underlying task immediately
        return tcs.Task;
    }
}

---

非常有趣。这肯定是有用的。我将看到其他答案，特别是那些使用任务并行库的答案。这是一个很好的技巧，让我能够更轻松地使用WebClient。谢谢

Task.Factory.StartNew(     () => OnFirstMessageReceived()  )
            .ContinueWith( () => OnSecondMessageReceived() )
            .ContinueWith( () => OnThirdMessageReceived()  )
            .ContinueWith( () => OnFourthMessageReceived() );

public static class Extensions  
{
    public static Task<XDocument> GetRssDownloadTask(
        this WebClient client, Uri rssFeedUri)
    {
        // task completion source is an object, which has some state.
        // it gives out the task, which completes, when state turns "completed"
        // or else it could be canceled or throw an exception
        var tcs = new TaskCompletionSource<XDocument>(); 

        // now we subscribe to completed event. depending on event result
        // we set TaskCompletionSource state completed, canceled, or error
        client.DownloadStringCompleted += (sender, e) => 
        {
                  if(e.Cancelled) 
                  {
                      tcs.SetCanceled();
                  }
                  else if(null != e.Error)
                  {
                      tcs.SetException(e.Error);
                  }
                  else
                  { 
                      tcs.SetResult(XDocument.Parse(e.Result));
                  }
        };

        // now we start asyncronous operation
        client.DownloadStringAsync(rssFeedUri);
        // and return the underlying task immediately
        return tcs.Task;
    }
}

public static void Main()
{
    var client = new WebClient();

    client.GetRssDownloadTask(
        new Uri("http://blogs.msdn.com/b/ericlippert/rss.aspx"))
        .ContinueWith( t => {
            ShowXmlInMyUI(t.Result); // show first result somewhere
            // start a new task here if you want a chain sequence
        });

    // or start it here if you want to get some rss feeds simultaneously

    // if we had await now, we would add 
    // async keyword to Main method defenition and then

    XDocument feedEric = await client.GetRssDownloadTask(
        new Uri("http://blogs.msdn.com/b/ericlippert/rss.aspx"));
    XDocument feedJon = await client.GetRssDownloadTask(
        new Uri("http://feeds.feedburner.com/JonSkeetCodingBlog?format=xml"));
    // it's chaining - one task starts executing after 
    // another, but it is still asynchronous
}