using System;

using System.Collections.Generic;

using System.Text;

using System.Threading;

using System.Threading.Tasks;

​

public class Runner

{

    int syncNum = 0;

    public async Task EnsureRunThingsExchangeAsync()

    {

        // x  replacement  compare

        if (Interlocked.Increment(ref syncNum) > 1)

        {

            Console.WriteLine("request rerun return");

            return;

        }

​

        do

        {

            Interlocked.Exchange(ref syncNum, 1);

            await ActualRunThingsAsync();

        }

        while (Interlocked.Decrement(ref syncNum) != 0);

        Console.WriteLine("end");

    }

​

    // one call runs things twice, and it can run concurrently

    public async Task EnsureRunThingsExchangeAsyncFlawed()

    {

        if (Interlocked.Increment(ref syncNum) > 1)

        {

            Console.WriteLine($"{DateTime.Now.ToString("HH:mm:ss.fff")} increment return");

            return;

        }

​

        do

        {

            await ActualRunThingsAsync();

        }

        while (Interlocked.Exchange(ref syncNum, 0) > 0);

        Console.WriteLine($"{DateTime.Now.ToString("HH:mm:ss.fff")} end");

    }

​

    public async Task ActualRunThingsAsync()

    {

        // do some things that should only happen one time at a time

        Console.WriteLine($"{DateTime.Now.ToString("HH:mm:ss.fff")} ActualRunThingsAsync");

        await Task.Delay(1000);

    }

}

​

public class Program

{

    public static async Task Main()

    {

        var r = new Runner();

        await Task.WhenAll(r.EnsureRunThingsExchangeAsync()

                          ,r.EnsureRunThingsExchangeAsync()

                           ,r.EnsureRunThingsExchangeAsync()

                           ,r.EnsureRunThingsExchangeAsync()

                          );

    }

}