using System;

using System.Threading.Tasks;

using System.Runtime.CompilerServices;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

    static async Task<int> GetNum1FromInternetAsync() { return 1; }

    static async Task<int> GetNum2FromInternetAsync() { return 2; }

    class GetSumAsyncCustomStateMachine : IAsyncStateMachine

    {

        enum Step

        {

            Beginning,

            AfterGotNum1,

            AfterGotNum2,

            Faulted = -1,

            Completed = -2,

        }

​

        public AsyncTaskMethodBuilder<int> TaskBuilder;

        TaskAwaiter<int> _awaiterHelper;

        Step _step;

        int _num1;

        int _num2;

​

        public GetSumAsyncCustomStateMachine()

        {

            _step = Step.Beginning;

        }

​

        public void MoveNext()

        {

            var me = this;

​

            if(_step == Step.Beginning)

            {

                _awaiterHelper = GetNum1FromInternetAsync().GetAwaiter();

                TaskBuilder.AwaitUnsafeOnCompleted(ref _awaiterHelper, ref me);

                _step = Step.AfterGotNum1;

            }

            else if(_step == Step.AfterGotNum1)

            {

                _num1 = _awaiterHelper.GetResult();

                _awaiterHelper = GetNum2FromInternetAsync().GetAwaiter();

                TaskBuilder.AwaitUnsafeOnCompleted(ref _awaiterHelper, ref me);

                _step = Step.AfterGotNum2;

            }

            else if(_step == Step.AfterGotNum2)

            {

                _num2 = _awaiterHelper.GetResult();

                TaskBuilder.SetResult(_num1 + _num2);

                _step = Step.Completed;

            }

        }

​

        public void SetStateMachine(IAsyncStateMachine m) {}

    }

}

​