using System;

using System.Threading.Tasks;

using System.Runtime.CompilerServices;

public class Program

{

    public static async Task Main()

    {

        await RunAsync();

    }

    [System.Runtime.CompilerServices.AsyncMethodBuilder(typeof(CustomTaskMethodBuilder))]

    public static async CustomTask RunAsync()

    {

        await Task.Yield();

        await default(CustomTask);

    }

}

​

public sealed class CustomTaskMethodBuilder

{

        public CustomTaskMethodBuilder()

            => Console.WriteLine("constructor");

​

        public static CustomTaskMethodBuilder Create()

            => new CustomTaskMethodBuilder();

​

        public void SetResult() => Console.WriteLine("SetResult");

​

        public void Start<TStateMachine>(ref TStateMachine stateMachine)

            where TStateMachine : IAsyncStateMachine

        {

            Console.WriteLine("Start task");

            stateMachine.MoveNext();

        }

​

        public CustomTask Task => default(CustomTask);

​

        public void AwaitOnCompleted<TAwaiter, TStateMachine>(

            ref TAwaiter awaiter, ref TStateMachine stateMachine)

            where TAwaiter : INotifyCompletion

            where TStateMachine : IAsyncStateMachine

        { }

​

        public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(

            ref TAwaiter awaiter, ref TStateMachine stateMachine)

            where TAwaiter : ICriticalNotifyCompletion

            where TStateMachine : IAsyncStateMachine

        { }

​

        public void SetException(Exception e) { }

​

        public void SetStateMachine(IAsyncStateMachine stateMachine) { }

    }

​

public struct CustomTask

{

    public CustomTaskAwaiter GetAwaiter() => default(CustomTaskAwaiter);

}

public struct CustomTaskAwaiter : INotifyCompletion

{

    public void GetResult() { }

    public bool IsCompleted => true;

    public void OnCompleted(Action continuation) { }

}