using System;

using System.Collections.Generic;

using System.Threading;

using System.Threading.Tasks;

public class Program

{

    public static async Task Main()

    {

        var service = new TokenService();

        Console.WriteLine(service.GetToken().AccessToken);

        await Task.Delay(1500); //make sure it is expired

        var requests = new List<Task>(10);

        for (int i = 0; i < 10; i++)

        {

            requests.Add(Task.Run(async() =>

            {

                await Task.Delay(Random.Shared.Next() % 1000);

                await service.RefreshToken();

                Console.WriteLine(service.GetToken().AccessToken);

            }));

        }

        await Task.WhenAll(requests);

    }

}

​

public class TokenService : ITokenService

{

    private readonly SemaphoreSlim semaphore = new(1);

    private DateTime lastRefreshed = DateTime.UtcNow;

    public Token GetToken()

        => new() { Scheme = "Bearer", AccessToken = lastRefreshed.ToString("HH:mm:ss")}; 

​

    public async Task RefreshToken()

    {

        await semaphore.WaitAsync();

​

        try

        {

            //Console.WriteLine($"{Thread.CurrentThread.ManagedThreadId} entered at: {DateTime.UtcNow}");

            //Use any arbitrary logic to detect simultaneous calls

            if((lastRefreshed - DateTime.UtcNow).Duration() < TimeSpan.FromSeconds(1).Duration()) 

            {

                Console.WriteLine("No refreshment happened");

                return;

            }

            Console.WriteLine("Refreshment happened");

            lastRefreshed = DateTime.UtcNow;

        }

        finally

        {

            semaphore.Release();

        }

    }

}

​

public record struct Token(string Scheme, string AccessToken);

​

public interface ITokenService

{

    Token GetToken();

    Task RefreshToken();

}