Hangs in custom AsyncTaskMethodBuilder.

[lsoft]

Hello!

I'm starting to deep dive into async machinery in C#, and I've implemented a custom AsyncTaskMethodBuilder, but it hangs in await. My CustomAsyncTaskMethodBuilder does nothing actually, and proxy all calls into default System.Runtime.CompilerServices.AsyncTaskMethodBuilder.

I don't understand why it hangs and where I made a mistake. Could someone highlight these points for me?

Thank you!

using System.Diagnostics;
using System.Runtime.CompilerServices;

await AwaitMethodCollection.TaskAsync(); //hang here

public static class AwaitMethodCollection
{
    [AsyncMethodBuilder(typeof(CustomAsyncTaskMethodBuilder))]
    public static async Task TaskAsync()
    {
        Console.WriteLine("TaskAsync");
        await Task.Delay(100);
    }
}

public readonly struct CustomAsyncTaskMethodBuilder
{
    private readonly AsyncTaskMethodBuilder _orig;

    public CustomAsyncTaskMethodBuilder()
    {
        _orig = new AsyncTaskMethodBuilder();
        Console.WriteLine("Builder: .ctor");
    }

    public static CustomAsyncTaskMethodBuilder Create()
    {
        return new CustomAsyncTaskMethodBuilder();
    }

    public Task Task => _orig.Task;

    public void SetResult()
    {
        _orig.SetResult();
        Console.WriteLine("Builder: SetResult");
    }

    public void SetException(Exception exception)
    {
        _orig.SetException(exception);
        Console.WriteLine("Builder: SetException");
    }

    public void AwaitOnCompleted<TAwaiter, TStateMachine>(
        ref TAwaiter awaiter,
        ref TStateMachine stateMachine)
        where TAwaiter : INotifyCompletion
        where TStateMachine : IAsyncStateMachine
    {
        _orig.AwaitOnCompleted(
           ref awaiter,
           ref stateMachine
           );
    }

    public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(
        ref TAwaiter awaiter,
        ref TStateMachine stateMachine)
        where TAwaiter : ICriticalNotifyCompletion
        where TStateMachine : IAsyncStateMachine
    {
        _orig.AwaitUnsafeOnCompleted(
           ref awaiter,
           ref stateMachine
           );
    }

    public void Start<TStateMachine>(ref TStateMachine stateMachine)
            where TStateMachine : IAsyncStateMachine
    {
        _orig.Start(ref stateMachine);
    }

    public void SetStateMachine(IAsyncStateMachine stateMachine)
    {
        _orig.SetStateMachine(stateMachine);
    }
}

its output:

Builder: .ctor
TaskAsync
Builder: SetResult

[VolodymyrBS]

You should remove readonly from _orig declaration.
AsyncTaskMethodBuilder is a mutable struct so the compiler makes a defensive copy of _orig before calling its method to make sure the field will not be changed. and so in the end _orig.Task will always return different task that will newer finish

[gfoidl]

...also remove readonly from the struct itself. The CustomAsyncTaskMethodBuilder needs to be a mutable struct itself.

That's the danger of mutable structs...there are a plethora of posts talking about "mutable structs being evil" -- so one has to be very cautious when using them. Also note that structs are passed by value (so a copy), thus take care to pass refs around in that case.

[lsoft]

arrgh! I'm aware about mutable structs, but forget that at this time! @VolodymyrBS @gfoidl thanks guys!