Cannot convert async lambda expression to delegate type 'Func<UIAccessibilityCustomAction, bool>

Question

Getting error:

Error CS4010: Cannot convert async lambda expression to delegate type 'Func'. An async lambda expression may return void, Task or Task, none of which are convertible to 'Func

UIAccessibilityCustomAction someAccessibilityAction = new
                UIAccessibilityCustomAction(
                SwipeActionMarkTextValue("Accessibility Text", false),
                            async (UIAccessibilityCustomAction arg) =>{

                                 await TestAsyncMethod();
                                return true;
                            });


    private async Task<bool> TestAsyncMethod()
    {
         await Task.Delay(5000);
        return true;
    }

And what don't you understand about the error? It's rather explicit in what it's saying is wrong. — Servy
@jimmyneutron You can not treat a Func<T, X> as asynchronous, it needs to be a synchronous behavior, your TestAsyncMethod would need to return bool — SushiHangover
@jimmyneutron No, it's returning a Task<bool> that will have a true result 5 seconds later. Big difference. — Servy

Marcell Toth Marcell Toth · Accepted Answer · 2018-11-06T19:22:17

Your async method doesn't return a bool. It returns a Task<bool>.

Remember that async is (kindof) syntactic sugar for ContinueWith. Your syntax is perfect but it helps to illustrate my point if I rewrite it as:

(UIAccessibilityCustomAction arg) => TestAsyncMethod().ContinueWith(b => true)

Possible solutions

I am not familiar with Xamarin.iOS, and it also depends on the task you want to achieve but here are some general solutions:

Use an API that accepts an async callback if there is one.
Make your work synchronous.

Wait (block) on your work item:

/*NO ASYNC HERE*/ (UIAccessibilityCustomAction arg) =>{
    TestAsyncMethod().GetAwaiter().GetResult();
        return true;
    })

Depending on your situation this might cause a deadlock though.

Make your worker "fire and forget" (async void) and immediately return true after kicking it off.

UIAccessibilityCustomAction someAccessibilityAction = new UIAccessibilityCustomAction(
    SwipeActionMarkTextValue("Accessibility Text", false),
    (UIAccessibilityCustomAction arg) => {
        TestAsyncMethod();
        return true;
    });
private async void TestAsyncMethod()
{
    await Task.Delay(5000);
}

Cannot convert async lambda expression to delegate type 'Func<UIAccessibilityCustomAction, bool>

1 Answers

Your async method doesn't return a bool. It returns a Task<bool>.

Possible solutions