How to use ThreadPool in c#?

Question

When I run this code then nothing is shown on the console, but when I debug then it displays the output. Please explain why this happen? How I can get info when the Thread completes the task?

public class TestClass
{
    static void Main()
    {
        ThreadPool.SetMaxThreads(5, 5);
        for (int x = 0; x < 10; x++)
        {
            ThreadPool.QueueUserWorkItem(new WaitCallback(printnum), x);
        }

        Console.ReadKey();
    }

    public static void printnum(object n)
    {
        Console.WriteLine("Call " + n);
        for (int i = 0; i < 10; i++) { Console.WriteLine(i); }
    }
}

Answer 1

The other's are right. If you do not wait for the threads to finish, you need to use Console.ReadLine. But if you do wait - as you asked - you can still use Console.ReadKey. I changed your code accordingly. Also checkout Microsofts example on how to use the ThreadPool.

    static void Main(string[] args)
    {
        const int count = 10;
        var waitHandles = new ManualResetEvent[count];

        ThreadPool.SetMaxThreads(5, 5);
        for (int x = 0; x < count; x++)
        {
            var handle = new ManualResetEvent(false);
            waitHandles[x] = handle;
            var worker = new MyWorker(handle, x);
            ThreadPool.QueueUserWorkItem(new WaitCallback(MyWorker.printnum), worker);
        }

        WaitHandle.WaitAll(waitHandles);

        Console.WriteLine("Press any key to finish");
        Console.ReadKey();
    }

    class MyWorker
    {
        readonly ManualResetEvent handle;
        readonly int number;

        public MyWorker(ManualResetEvent handle, int number)
        {
            this.handle = handle;
            this.number = number;
        }

        public static void printnum(object obj)
        {
            var worker = (MyWorker)obj;
            Console.WriteLine("Call " + worker.number);
            for (int i = 0; i < 10; i++) { Console.WriteLine(i); }

            // we are done
            worker.handle.Set();
        }
    }

The key is that you have to use WaitHandles. Each thread gets one handle which is set to true when the thread finishes. In your main method you have to wait for all handles to be set to true.

Answer 2

From the documentation for Console.ReadKey():

The ReadKey method waits, that is, blocks on the thread issuing the ReadKey method, until a character or function key is pressed.

What it actually does is acquire a lock on Console.InternalSyncObject, which prevents further operations on the console.

The Console.ReadLine() method does not block the thread in this way. You can use it instead.

Reading this article I'm guessing you have .NET 4.5 installed?