Async WebApi ActionFilterAttribute. An asynchronous module or handler completed while an asynchronous operation was still pending

actionfilterattributeasp.net-web-apiasync-await

I understand await waits for a task (an awaitable) to complete.
But I'm confused about what that actually means.

The code that doesn't work:

public async override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
{
    if (actionExecutedContext.Response.Content != null)
    {
        var responseContent = await actionExecutedContext.Response.Content.ReadAsStringAsync();
        DoSomething(responseContent);
    }
}

The code that does work:

public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
{
    if (actionExecutedContext.Response.Content != null)
    {
        var responseContent = actionExecutedContext.Response.Content.ReadAsStringAsync().ContinueWith(
        task =>
        {
            DoSomething(task.Result);
        });
    }
}

Obviously the error message An asynchronous module or handler completed while an asynchronous operation was still pending. tells me that there was no waiting for the async call to complete but instead the "main" thread continued. I expected the thread to continue but not within the current method. I thought the thread would return to the asp.net stack do some other work and return once the await asyncOperation() operation completed.

I'm using await in other places too – (e.g. waiting for web service responses) – and I didn't run into similar problems anywhere. I wonder why the IActionFilterAttribute behaves differently. In fact my web service calls probably take way longer than reading the content of the response into a string.

Can someone please enlighten me? I have the feeling I didn't understand the concept.

Best Answer

Adding async code to a method that returns void is dangerous and almost never what you actually want to do. See What's the difference between returning void and returning a Task?.

Instead, you need to override/implement a method that returns a task. In this case, ActionFilterAttribute hides the Task that IHttpActionFilter provides, so you'll need to implement IActionFilter (ExecuteActionFilterAsync) instead. If you want to use you code as an attribute, just make sure you also derive from the Attribute class.

For example:

public class AsyncActionFilterAttribute : Attribute, IActionFilter
{
    public async Task<HttpResponseMessage> ExecuteActionFilterAsync(HttpActionContext actionContext, CancellationToken cancellationToken, Func<Task<HttpResponseMessage>> continuation)
    {
        HttpResponseMessage response = await continuation();
        DoSomething(response);
        return response;
    }
}

Related Solutions

C# – Web Api + HttpClient: An asynchronous module or handler completed while an asynchronous operation was still pending

Your problem is a subtle one: the async lambda you're passing to PushStreamContent is being interpreted as an async void (because the PushStreamContent constructor only takes Actions as parameters). So there's a race condition between your module/handler completing and the completion of that async void lambda.

PostStreamContent detects the stream closing and treats that as the end of its Task (completing the module/handler), so you just need to be sure there's no async void methods that could still run after the stream is closed. async Task methods are OK, so this should fix it:

private static PushStreamContent CopyContentStream(HttpResponseMessage sourceContent)
{
  Func<Stream, Task> copyStreamAsync = async stream =>
  {
    using (stream)
    using (var sourceStream = await sourceContent.Content.ReadAsStreamAsync())
    {
      await sourceStream.CopyToAsync(stream);
    }
  };
  var content = new PushStreamContent(stream => { var _ = copyStreamAsync(stream); });
  return content;
}

If you want your proxies to scale a bit better, I also recommend getting rid of all the Result calls:

//Controller entry point.
public async Task<HttpResponseMessage> PostAsync()
{
  using (var client = new HttpClient())
  {
    var request = BuildRelayHttpRequest(this.Request);

    //HttpCompletionOption.ResponseHeadersRead - so that I can start streaming the response as soon
    //As it begins to filter in.
    var relayResult = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);

    var returnMessage = BuildResponse(relayResult);
    return returnMessage;
  }
}

Your former code would block one thread for each request (until the headers are received); by using async all the way up to your controller level, you won't block a thread during that time.

C# – How to safely call an async method in C# without await

If you want to get the exception "asynchronously", you could do:

  MyAsyncMethod().
    ContinueWith(t => Console.WriteLine(t.Exception),
        TaskContinuationOptions.OnlyOnFaulted);

This will allow you to deal with an exception on a thread other than the "main" thread. This means you don't have to "wait" for the call to MyAsyncMethod() from the thread that calls MyAsyncMethod; but, still allows you to do something with an exception--but only if an exception occurs.

Update:

technically, you could do something similar with await:

try
{
    await MyAsyncMethod().ConfigureAwait(false);
}
catch (Exception ex)
{
    Trace.WriteLine(ex);
}

...which would be useful if you needed to specifically use try/catch (or using) but I find the ContinueWith to be a little more explicit because you have to know what ConfigureAwait(false) means.

Best Answer

Related Solutions

C# – Web Api + HttpClient: An asynchronous module or handler completed while an asynchronous operation was still pending

C# – How to safely call an async method in C# without await

Update:

Related Topic