Wait on a task invoked in a delegate

Question

I have a controller method which looks something like this:

[HttpPut, Route("cqs/command")]
public HttpResponseMessage Command([ValueProvider(typeof(HeaderValueProviderFactory))] string typeName)
{
    object reply = null;
    var code = HttpStatusCode.OK;
    try
    {
        var cmdObject = DeserializeRequest(typeName);
        var method = _commandMethod.MakeGenericMethod(type);
        method.Invoke(this, new object[] { request });
    }
    catch (Exception exception)
    {
        code = HttpStatusCode.InternalServerError;
        reply = exception;
    }

    var responseMsg = new HttpResponseMessage(code);
    if (reply != null)
    {
        responseMsg.Headers.Add("X-TypeName", reply.GetType().AssemblyQualifiedName);
        var replyJson = JsonConvert.SerializeObject(reply);
        responseMsg.Content = new StringContent(replyJson, Encoding.UTF8, "application/json");
    }
    return responseMsg;
}

Which invokes the following method:

private void ExecuteCommand<T>(T command) where T : Command
{
    var task = _commandBus.ExecuteAsync(command);
    task.Wait(TimeSpan.FromSeconds(10));
}

The reason to that is that the _commandBus only have a generic method which I need to invoke.

The problem is however that the ExecuteCommand seems to deadlock at some times. I can't figure out why. The ICommandBus.ExecuteAsync method will invoke other tasks using async/await, so it might be some kind of dead lock since WebApi uses a synchronization context? (await vs Task.Wait - Deadlock?)

So if I understand it all correctly, there might be two solutions:

use async/await all the way. But how do I do that when invoking a method using MethodInfo?
Change so that my invocation works with the synchronization context

I'm lost when it comes to both solutions. Can anyone help?

The problem doesn't seem to come from invoking the delegate though, it comes from the fact that you're doing a blocking wait on the task, when you should be asynchronously waiting. — Servy, Mar 03 '14 at 20:24

score 5 · Accepted Answer · answered Mar 03 '14 at 20:47

5

Change ExecuteCommand<T> so that it looks like this (I'm assuming your actual code will do something on a timeout):

private async Task ExecuteCommandAsync<T>(T command) where T : Command
{
  var timeout = Task.Delay(TimeSpan.FromSeconds(10));
  var task = _commandBus.ExecuteAsync(command);
  await Task.WhenAny(task, timeout);
}

And make whatever calls it async Task as well. Then, when you invoke the method, you can do this:

var task = method.Invoke(this, new object[] { request }) as Task;
await task;

answered Mar 03 '14 at 20:47

Stephen Cleary

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That works for the Command execution. But the Query variant returns a generic task. What is the best way to handle that? – jgauffin Mar 03 '14 at 20:50
2

`Task` inherits from `Task`, so if you just want to `await` it you don't need to do anything else. If you need to return a result somehow, I'd recommend `dynamic`. – Stephen Cleary Mar 03 '14 at 20:55

Wait on a task invoked in a delegate

1 Answers1