6

I think I missunderstanding the behaviour of async await in c#.

I have two methods that return a Task defined like

public async Task Name()
{
await AsyncOperation() 
}

Imagine AsyncOperation() like an PostAsync of HttpClient.

Now I call them inside some other methods

public asyn Task Method()
{
await Name1(() => { "bla bla"});
await Name2();
Console.WriteLine("Continue");
}

This works as expected to me. Waits until Name1() and Name2() finish and then continues.

Now I need to nest Name1() and Name2(). In fact Name1() is a Please Wait Window that recieve as lambda parameters a slow operation, while Name2() is a slow download of a file. I want the Plese Wait window appears while the file is downloaded.

So I try something like this:

public asyn Task Method()
{
    await Name1( async ()=>
    {
        await Name2();
    }
    Console.WriteLine("Continue");
}

In this case the execution doesnt wait untile Name2() finished. Why this happen and await doesnt wait?

Update

This is the logic behind the method of please wait. It shows a Please Wait message using Mahapps Dialogs, executes the code that recieves by the lambda, and then close the please wait message.

public static async Task Name1(Action longOperation)
{
    _progressController = await _metroWindow.ShowProgressAsync("Please wait...");            
    await Task.Run(() => longOperation());
    await _progressController.CloseAsync();
}
Ricardo Polo Jaramillo
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1 Answers1

7

The Name1 method takes a delegate and returns a Task<T> where T is the type returned by the delegate. In your case, the delegate returns Task, so we get Task<Task> as the result. Using await waits only for the completion of the outer task (which immediately returns the inner task) and the inner task is then ignored.

You can fix this by dropping the async and await in the lambda function.

Also, take a look at Asynchronous Gotchas in C#.

djikay
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    Couldn't he revise his `Task.Run` to `await Task.Run(async() => await longOperation());` to return control back to the calling member until the long operation completes? This way his await carries all the way, including the nested await? – Johnathon Sullinger Jul 13 '14 at 02:53
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    @JohnathonSullinger, if `longOperation` is already *async* (returns a `Task`), you very rarely need to wrap it with `Task.Run`. It'd only make sense if there's a chunk of blocking or CPU-intensive code at the beginning of `longOperation`, or if you want to execute it without sync. context. If you do wrap it, `await Task.Run(() => longOperation())` would be enough (no need for an `async` lambda). – noseratio Jul 13 '14 at 03:38