Use ThreadPool in C# within a loop and wait for all threads to finish

Question

I have a simple code that looks like this in C#:

using System;
using System.Threading;
using System.Diagnostics;


namespace ThreadPooling
{
    class Program
    {
        static void Main(string[] args)
        {

            Console.WriteLine("Enter the number of calculations to be made:");
            int calculations= Convert.ToInt32(Console.ReadLine());

            Console.WriteLine("Thread Pool Execution");

            for (int i = 1; i <= calculations; i++)
            {
                Console.WriteLine("Staring process " + i + "...");
                ThreadPool.QueueUserWorkItem(new WaitCallback(Process(i)));
            }

            Console.WriteLine("All calculations done.");
            Console.WriteLine("\nPress any key to exit the program...");
            Console.ReadKey();

        }

        static void Process(object callback, int name)
        {
            for (int i = 0; i <= 10; i++)
            {
                Console.WriteLine(i + " is the current number in " +  name);
            }
        }


    }
}

I want main to be able to call Process with Thread Pool and an argument. I then want the program to wait for all threads to finish before telling the user that it is done. How do I do both? It seems I cannot put an argument inside Process, I get: Error CS7036 There is no argument given that corresponds to the required formal parameter 'name' of 'Program.Process(object, int)'

And I am not clear on how to tell C# to wait for all processes with in the loop to finish before it tells the user it is done.

Your method `Process` takes two parameters, `object` and `int`, but when you call it you are only passing in one argument. Try `ThreadPool.QueueUserWorkItem(_ => Process(i, i));` — Chronicle, Apr 13 '20 at 22:13

score 2 · Accepted Answer · answered Apr 13 '20 at 22:14

.NET is almost 20 years old, and has several generations of improving APIs in it.

This is trivial with the newer Task Parallel Library methods. EG

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace ThreadPooling
{
    class Program
    {
        static void Main(string[] args)
        {

            Console.WriteLine("Enter the number of calculations to be made:");
            int calculations = Convert.ToInt32(Console.ReadLine());

            var tasks = new List<Task>();
            for (int i = 1; i <= calculations; i++)
            {
                int processNum = i;
                Console.WriteLine("Staring process " + processNum + "...");
                var task = Task.Run(() => Process(processNum));
                tasks.Add(task);
            }

            Task.WaitAll(tasks.ToArray());
            Console.WriteLine("All calculations done.");
            Console.WriteLine("\nPress any key to exit the program...");
            Console.ReadKey();

        }

        static void Process(int name)
        {
            for (int i = 0; i <= 10; i++)
            {
                Console.WriteLine(i + " is the current number in " + name);
            }
        }


    }
}

Thanks works! As a side note: does C++ have ThreadPooling too? — KGB91, Apr 13 '20 at 22:18
There's not one built-in to the C++ language, but Windows does. And the .NET QueueUserWorkItem API was inspired by it. See eg: https://learn.microsoft.com/en-us/windows/win32/api/threadpoollegacyapiset/nf-threadpoollegacyapiset-queueuserworkitem?redirectedfrom=MSDN — David Browne - Microsoft, Apr 13 '20 at 22:41

Use ThreadPool in C# within a loop and wait for all threads to finish

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