Creating an typed async Task

Question

I want to create some async Tasks without starting them at the moment. With the untyped Tasks there is no problem:

    private Task CreateTask() {
        return new Task(
             async () => await DoSomething().ConfigureAwait(false));
    }

But to handle Exceptions I added a return value to my function. But how can I write it in a correct syntax:

    private Task<bool> CreateTask() {
        return new Task<bool>(
            async () =>  await DoSomething().ConfigureAwait(false));
    }

I get the message, that the async lambda expression can't be converted to func. So my question: How is it written correctly?

Both methods are wrong. DoSomething() already returns a running Task so you can just return it as-is, return (Task)DoSomething();. await awaits for an already running task to complete, it doesn't make it start asynchronously. There's no reason to create a cold task with new. In the second case, what is the value supposed to be? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Dec 9, 2016 at 13:04
This sounds like a case of the XY Problem. You have a problem with X, think that Y is the solution and ask about Y when it doesn't work. What is the actual problem you want to solve? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Dec 9, 2016 at 13:05
To handle exceptions enclose your await call with an exception handler, just as you would with a synchronous method. await will cause the original exception to be thrown. You could write eg try{ .... await CallMyServerAsync();}catch(WebException exc){...} just as you would write try{ .... await CallMyServer();}catch(WebException exc){...} if CallMyServer is a blocking method — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Dec 9, 2016 at 13:12

Stephen Cleary · Accepted Answer · 2016-12-09 16:17:19Z

3

I want to create some async Tasks without starting them at the moment.

That is the wrong solution for whatever problem you're trying to solve. If you want to define code that you want to run in the future, then you should use a delegate:

private Func<Task> CreateTaskFactory()
{
  return async () => await DoSomething().ConfigureAwait(false);
}

But to handle Exceptions I added a return value to my function.

Again, that's not the best solution. Tasks already understand exceptions just fine without writing any additional code.

If you do need to return a value (as data), then you can return a delegate that creates a Task<T>:

private Func<Task<int>> CreateTaskFactory()
{
  return async () => await DoSomethingReturningInt().ConfigureAwait(false);
}

answered Dec 9, 2016 at 16:17

Stephen Cleary

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5 Comments

Servy Over a year ago

CreateTaskFactory can just be return DoSomething(); There's no advantage to an async delegate here.

Stephen Cleary Over a year ago

@Servy: I'm assuming that the op wants to define the operation now, but start it at a later time.

Servy Over a year ago

Sorry, meant return () => DoSomething();

Stephen Cleary Over a year ago

@Servy: Oh, yeah. Absolutely, unless the op's delegates are not trivial.

Servy Over a year ago

Sure, although if it's non-trivial enough it would most likely best be refactored out into a named method.

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