C# Online Compiler | .NET Fiddle

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) { }
}

 
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");
​