using System;

using System.Threading;

using System.Threading.Tasks;

using System.Collections.Concurrent;

public class Program

{

    public class JobDistributor

    {

        public Task? RunningTask {get; private set;}

        DateTime? TimeOfStart {get; set;}

​

        readonly SemaphoreSlim _semaphore = new(1,1);

        public async Task<Task> StartOrGetTheRunningJobAsync()

        {

            var requestedStartDate = DateTime.UtcNow;

            while(true){

                if (RunningTask is null || RunningTask.IsCompleted)

                {

                    try{

                        await _semaphore.WaitAsync();

                        Console.WriteLine($"Semaphore acquired");

                        if (RunningTask is null || RunningTask.IsCompleted)

                        {

                            RunningTask = RunJobAsync();

                            TimeOfStart = DateTime.UtcNow;

                            return RunningTask;

                        }

                    }

                    finally{

                        _semaphore.Release();

                    }

                }

                if (RunningTask is not null && !RunningTask.IsCompleted && TimeOfStart.Value.Ticks >= requestedStartDate.Ticks){

                    return RunningTask;

                }

                Console.WriteLine($"Old task is running, waiting...");

                await Task.Delay(100);

            }

        }

    }

    private static readonly ConcurrentDictionary<int, JobDistributor> _jobs = new()

    {

        [0] = new()

    };

    public static async Task Main()

    {

        var jobDistributor = _jobs[0];

        jobDistributor.StartOrGetTheRunningJobAsync();

        await Task.Delay(100);

​

        Task[] sameTimeRequests = new[]

        {

            jobDistributor.StartOrGetTheRunningJobAsync(),

            jobDistributor.StartOrGetTheRunningJobAsync(),

            jobDistributor.StartOrGetTheRunningJobAsync(),

            jobDistributor.StartOrGetTheRunningJobAsync()

        };

​

        Task.WaitAll(sameTimeRequests);

        await Task.Delay(100);

        sameTimeRequests = new[]

        {

            jobDistributor.StartOrGetTheRunningJobAsync(),

            jobDistributor.StartOrGetTheRunningJobAsync(),

            jobDistributor.StartOrGetTheRunningJobAsync(),

            jobDistributor.StartOrGetTheRunningJobAsync()

        };

        Task.WaitAll(sameTimeRequests);

​

        await jobDistributor.RunningTask;

    }

    public static async Task RunJobAsync(){

        await Task.Delay(1000);

        Console.WriteLine($"Finished process: {Guid.NewGuid()}");

    }

​

    public async Task BasicUsageWouldBe()

    {

        var runningJobTask = await _jobs[0].StartOrGetTheRunningJobAsync();

        await runningJobTask;

        // return or whatever

    }

}