How is the mechanism for waiting for task completion within a thread pool implemented?

In C#, the mechanism for waiting for tasks to complete within a thread pool is usually implemented using the Task class and synchronization methods such as Task.Wait(), Task.WaitAll(), or Task.WhenAll().

When tasks are started in a thread pool via Task.Run or ThreadPool.QueueUserWorkItem, you can wait for their completion by calling:

Task[] tasks = new Task[3];
tasks[0] = Task.Run(() => { /* work */ });
tasks[1] = Task.Run(() => { /* work */ });
tasks[2] = Task.Run(() => { /* work */ });

// Wait for all tasks to complete
Task.WaitAll(tasks);

If ThreadPool is used directly, you can apply ManualResetEvent or CountdownEvent for waiting:

CountdownEvent countdown = new CountdownEvent(3);

for (int i = 0; i < 3; i++)
{
    ThreadPool.QueueUserWorkItem(_ => {
        // work
        countdown.Signal(); // decrement the counter
    });
}

// Wait for all tasks to complete
countdown.Wait();

Thus, the waiting mechanism is based on thread synchronization and the use of special methods and classes to block the current thread until all launched tasks are completed.