ThreadPool

Learn the thread pool with ThreadPool.

ThreadPool

The ThreadPool is a pool of ready threads shared by short tasks in the application. Creating a new Thread each time is costly; the ThreadPool reuses its threads.

ThreadPool.QueueUserWorkItem(callback) enqueues a work item that runs on an idle pool thread. The ThreadPool is not suitable for long work; it can exhaust the pool.

ThreadPool and waiting

There is no guarantee when work enqueued to the ThreadPool will finish. In tests you wait with a signal mechanism such as ManualResetEvent or CountdownEvent.

Production code usually prefers Task.Run; it also uses the ThreadPool but makes waiting easy with await and keeps the code simple.

Exercise ThreadPool with a signal wait

Create a signal with ManualResetEvent done = new ManualResetEvent(false);. Enqueue work with ThreadPool.QueueUserWorkItem(_ => { done.Set(); });. Wait with done.WaitOne();. Finally print Console.WriteLine("Done");.

Show example solution

ManualResetEvent done = new ManualResetEvent(false); ThreadPool.QueueUserWorkItem(_ => { done.Set(); }); done.WaitOne(); Console.WriteLine("Done");
Example solution:
using System;
using System.Threading;

class Program
{
    static void Main()
    {
        ManualResetEvent done = new ManualResetEvent(false);
        ThreadPool.QueueUserWorkItem(_ => { done.Set(); });
        done.WaitOne();
        Console.WriteLine("Done");
    }
}

Test yourself

What does ThreadPool.QueueUserWorkItem do?
- Creates a new thread
- Cancels a thread
- Pauses a thread
- Enqueues a work item
Show answer

Enqueues a work item — Correct. QueueUserWorkItem enqueues a work item.

The QueueUserWorkItem call

ThreadPool.____(state => { /* iş */ });

Show answer

ThreadPool.QueueUserWorkItem(state => { /* iş */ });

Match the ThreadPool concepts

ThreadPool

Ready thread pool
QueueUserWorkItem

Enqueue work
ManualResetEvent

Wait with signal
Task.Run

ThreadPool + await

Next lesson: The lock statement

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