今天我读了很多关于 async/await 的文章,这让我大吃一惊。 我不明白为什么下面的测试通过了。
[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。我怎样才能得到这种行为?
最佳答案
让你的方法async Task
而不是async void
,让你的测试方法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();
}
有几个充分的理由可以避免 async void
。错误处理就是其中之一。 async void
方法引发的错误直接进入该方法启动时的当前 SynchronizationContext
。
您的测试通过的原因是 async
方法可能在完成之前返回给调用者。测试运行器看到测试方法返回(还没有抛出异常),并将其标记为“通过”。如果您从测试方法返回 Task
,则测试运行器知道要等待 Task
完成,然后再考虑测试完成。
关于c# - 如何从异步方法向调用者抛出异常?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14379845/