如何从异步方法向调用者抛出异常？

Question

今天我读了很多关于async/await的内容,这让我大吃一惊.我无法理解为什么以下测试通过.

[Test]
public void Test()
{
    var listener = new AsyncHttpListener();
    listener.ListeningAsync();

    try
    {
        new WebClient().DownloadString("http://localhost:8080/");
    }
    catch (Exception)
    {
    }

    listener.Close();
}

public class AsyncHttpListener
{
    private readonly HttpListener listener;

    public AsyncHttpListener()
    {
        listener = new HttpListener();
        listener.Prefixes.Add("http://localhost:8080/");
        listener.Start();
    }

    public void Close()
    {
        listener.Close();
    }

    public async void ListeningAsync()
    {
        var context = await listener.GetContextAsync();
        HandleContext(context);
    }

    private void HandleContext(HttpListenerContext context)
    {
        throw new Exception("test excpetion");
    }
}

测试通过,但输出包含:

System.Exception
test excpetion
   at AsyncHttpListenerTest.AsyncHttpListener.HandleContext(HttpListenerContext context) in AsyncHttpListener.cs: line 30
   at AsyncHttpListenerTest.AsyncHttpListener.d__0.MoveNext() in AsyncHttpListener.cs: line 25
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.b__1(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

我希望异常将从任务线程(HandleContext()方法)传输到调用者上下文并且测试失败.我怎么能得到这种行为？

Answer 1

不要使用您的方法,async Task而是async void使用您的测试方法async Task而不是void:

public async Task ListeningAsync()
{
    var context = await listener.GetContextAsync();
    HandleContext(context);
}

[Test]
public async Task Test()
{
    var listener = new AsyncHttpListener();
    await listener.ListeningAsync();

    try
    {
        new WebClient().DownloadString("http://localhost:8080/");
    }
    catch (Exception)
    {
    }

    listener.Close();
}

有几个很好的理由可以避免async void.错误处理就是其中之一.从async void方法中引发的错误直接转到SynchronizationContext方法启动时的当前错误.

测试通过的原因是因为async方法可能会在完成之前返回给调用者.测试运行器看到测试方法返回(没有抛出异常),并将其标记为"已通过".如果Task从测试方法返回,那么测试运行器知道Task在考虑测试完成之前等待完成.