小心C# 5.0 中的await and async模式造成的死锁

平时在使用C# 5.0中的await and async关键字的时候总是没注意，直到今天在调试一个ASP.NET项目时，发现在调用一个声明为async的方法后，程序老是莫名其妙的被卡住，就算声明为async的方法中的Task任务执行完毕后，外部方法的await调用还是阻塞着，后来查到了下面这篇文章，才恍然大悟原来await and async模式使用不当很容易造成程序死锁，下面这篇文章通过一个Winform示例和一个Asp.net示例介绍了await and async模式是如何造成程序死锁的，以及如何避免这种死锁。

原文链接

UI Example

Consider the example below. A button click will initiate a REST call and display the results in a text box (this sample is for Windows Forms, but the same principles apply to any UI application).

// My "library" method.
public static async Task<JObject> GetJsonAsync(Uri uri)
{
  using (var client = new HttpClient())
  {
    var jsonString = await client.GetStringAsync(uri);
    return JObject.Parse(jsonString);
  }
}

// My "top-level" method.
public void Button1_Click(...)
{
  var jsonTask = GetJsonAsync(...);
  textBox1.Text = jsonTask.Result;
}

The “GetJson” helper method takes care of making the actual REST call and parsing it as JSON. The button click handler waits for the helper method to complete and then displays its results.

This code will deadlock.

ASP.NET Example

This example is very similar; we have a library method that performs a REST call, only this time it’s used in an ASP.NET context (Web API in this case, but the same principles apply to any ASP.NET application):

// My "library" method.
public static async Task<JObject> GetJsonAsync(Uri uri)
{
  using (var client = new HttpClient())
  {
    var jsonString = await client.GetStringAsync(uri);
    return JObject.Parse(jsonString);
  }
}

// My "top-level" method.
public class MyController : ApiController
{
  public string Get()
  {
    var jsonTask = GetJsonAsync(...);
    return jsonTask.Result.ToString();
  }
}

This code will also deadlock. For the same reason.

What Causes the Deadlock

Here’s the situation: remember from my intro post that after you await a Task, when the method continues it will continue in a context.

In the first case, this context is a UI context (which applies to any UI except Console applications). In the second case, this context is an ASP.NET request context.

One other important point: an ASP.NET request context is not tied to a specific thread (like the UI context is), but it does only allow one thread in at a time. This interesting aspect is not officially documented anywhere AFAIK, but it is mentioned in my MSDN article about SynchronizationContext.

So this is what happens, starting with the top-level method (Button1_Click for UI / MyController.Get for ASP.NET):

1. The top-level method calls GetJsonAsync (within the UI/ASP.NET context).
2. GetJsonAsync starts the REST request by calling HttpClient.GetStringAsync (still within the context).
3. GetStringAsync returns an uncompleted Task, indicating the REST request is not complete.
4. GetJsonAsync awaits the Task returned by GetStringAsync. The context is captured and will be used to continue running the GetJsonAsync method later. GetJsonAsync returns an uncompleted Task, indicating that the GetJsonAsync method is not complete.
5. The top-level method synchronously blocks on the Task returned by GetJsonAsync. This blocks the context thread.
6. … Eventually, the REST request will complete. This completes the Task that was returned by GetStringAsync.
7. The continuation for GetJsonAsync is now ready to run, and it waits for the context to be available so it can execute in the context.
8. Deadlock. The top-level method is blocking the context thread, waiting for GetJsonAsync to complete, and GetJsonAsync is waiting for the context to be free so it can complete.

For the UI example, the “context” is the UI context; for the ASP.NET example, the “context” is the ASP.NET request context. This type of deadlock can be caused for either “context”.

Preventing the Deadlock

There are two best practices (both covered in my intro post) that avoid this situation:

1. In your “library” async methods, use ConfigureAwait(false) wherever possible.
2. Don’t block on Tasks; use async all the way down.

这里我补充一下，如果你开发的是Winform程序，那么最好用第二种方法避免死锁，也就是不要阻塞主线程（也就是本文中提到的context thread），这样当await等待的Task对象线程执行完毕后，由于主线程没有被阻塞，因此await后面的代码就会继续在主线程上执行完毕。之所以在Winform中不推荐用第一种方法是因为第一种方法会让await后面的代码在一个新的线程上执行的，如果await后有代码设置了Winform控件的值，那么会引起Winform程序的线程安全问题，所以在Winform中最好的办法还是不要阻塞主线程，让await后面的代码能够在主线程上执行。但在Asp.net中用上面第一种或第二种方法都可以，不存在线程安全问题。

Consider the first best practice. The new “library” method looks like this:

public static async Task<JObject> GetJsonAsync(Uri uri)
{
  using (var client = new HttpClient())
  {
    var jsonString = await client.GetStringAsync(uri).ConfigureAwait(false);
    return JObject.Parse(jsonString);
  }
}

This changes the continuation behavior of GetJsonAsync so that it does not resume on the context. Instead, GetJsonAsync will resume on a thread pool thread. This enables GetJsonAsync to complete the Task it returned without having to re-enter the context.

Consider the second best practice. The new “top-level” methods look like this:

public async void Button1_Click(...)
{
  var json = await GetJsonAsync(...);
  textBox1.Text = json;
}

public class MyController : ApiController
{
  public async Task<string> Get()
  {
    var json = await GetJsonAsync(...);
    return json.ToString();
  }
}

This changes the blocking behavior of the top-level methods so that the context is never actually blocked; all “waits” are “asynchronous waits”.

Note: It is best to apply both best practices. Either one will prevent the deadlock, but both must be applied to achieve maximum performance and responsiveness.

最后再补充说一点，本文提到的await and async死锁问题，在.Net控制台程序中并不存在。因为经过实验发现在.Net控制台程序中，await后面的代码默认就是在一个新的线程上执行的，也就是说在控制台程序中就算不调用Task.ConfigureAwait(false)，await后面的代码也会在一个新启动的线程上执行，不会和主线程发生死锁。但是在Winform和Asp.net中就会发生死锁。