Example of checking the number of threads in a thredpool

ThreadPool with counter.cs

        [Theory]
        [InlineData(1500, 300)]
        [InlineData(1200, 160)]
        public void ThreadPoolWithCounter(int booksAllowance, int threadsToRun)
        {
            // Initialize counter with 1, otherwise, the AddCount method will throw an error (the counter is signaled and cannot be incremented).
            using (CountdownEvent cde = new CountdownEvent(1))
            {
                ManualResetEvent manualResetEvent = new ManualResetEvent(false);

                LibraryAccount account = new LibraryAccount(booksAllowance: booksAllowance);

                for (int i = 0; i < threadsToRun; i++)
                {
                    bool enqueued = ThreadPool.QueueUserWorkItem(delegate
                   {
                    // Increase the counter
                    cde.AddCount();
                    Trace.WriteLine(message: $"Increase CountDown, current count: {cde.CurrentCount}");
                    // Execute the business logic: a thread-safe withdraw books (implements lock internally)
                    account.WithdrawBooksWithExecuterName(10, "Thread_" + DateTime.Now.Ticks);
                    // Decrease the counter
                    cde.Signal();
                    Trace.WriteLine(message: $"Decrease CountDown, current count: {cde.CurrentCount}");
                    // Signal that at least one thread is executed.
                    manualResetEvent.Set();
                   });
                }

                // ensure that at least one thread was created 
                manualResetEvent.WaitOne();

                // Decrease the counter (as it was initialized with the value 1).
                cde.Signal();

                // Wait until the counter is zero.
                cde.Wait();

                Trace.WriteLine(message: $"All threads are finished, current count: {cde.CurrentCount}");
            }
        }