Terminate or exit C# Async method with "return"

Question

I was new to the async-await method in C# 5.0, and I have few questions in my mind

What is the best way to escape an async method if it failed an input argument or null check?
What is the logical flow of using return; in an Task async method (In some circumstances, it became an infinite loop)?
Is CancellationToken or Task.Yield fit better in this scenario?

public Func<AzureBlobInfo, string, Task> UploadSuccessCallBackAsync { get; set; }

private async Task OnUploadSuccessAsync(AzureBlobInfo info)
{
    if (this.UploadSuccessCallBackAsync == null)
    {
        return;
    }

    var transactionType = this.FormData.Get("transactionType");
    if (string.IsNullOrEmpty(transactionType))
    {
        transactionType = "unknown";
    }

    await this.UploadSuccessCallBackAsync(info, transactionType);
}

As a general rule, you write an `async` method just like you would write a synchronous method. Return when the method is done; throw exceptions for errors. — Stephen Cleary, Jul 31 '14 at 12:51

i3arnon · Accepted Answer · 2014-07-31T10:13:07.213

The best way to "fail upon some problem" IMHO would be to throw the appropriate exception, but you can definitely just use return; if you prefer to avoid exceptions.

This will create a completed/faulted task that was completed synchronously, so the caller using await will get a finished task and continue on using the same thread.

CancellationToken allows for the caller to cancel the operation, which isn't the case you are describing.
Task.Yield doesn't terminate any operation, it just enables other tasks to run for some time and reschedules itself for later.

score 2 · Answer 2 · answered Jul 31 '14 at 09:53

2

You can safely return from an async method at any moment. In your case (method returning a Task) the compiler will generate a terminated task, so any caller awaiting on your function will continue.

answered Jul 31 '14 at 09:53

Thierry

342
2
6

score 0 · Answer 3 · edited May 23 '17 at 11:59

When throwing an exception from an async method it gets captured in the task so it does not get thrown until the task is observed by calling Task.Wait(), Task.Result, awaiting the task or accessing the Exceptions property of the task.

In terms of input arguments one way to get around that is to split the method into two parts, the first checks the input arguments and then calls the second method which is async. See this questions answer by Stephen Cleary for an example. That way the input argument exception will be thrown straight away outside of the task returned by the method.

Terminate or exit C# Async method with "return"

3 Answers3