How to: Use a Thread Pool (C# and Visual Basic)

Thread pooling is a form of multithreading in which tasks are added to a queue and automatically started when threads are created. For more information, see Thread Pooling (C# and Visual Basic).

The following example uses the .NET Framework thread pool to calculate the Fibonacci result for ten numbers between 20 and 40. Each Fibonacci result is represented by the Fibonacci class, which provides a method named ThreadPoolCallback that performs the calculation. An object that represents each Fibonacci value is created, and the ThreadPoolCallback method is passed to QueueUserWorkItem, which assigns an available thread in the pool to execute the method.

Because each Fibonacci object is given a semi-random value to compute, and because each thread will be competing for processor time, you cannot know in advance how long it will take for all ten results to be calculated. That is why each Fibonacci object is passed an instance of the ManualResetEvent class during construction. Each object signals the provided event object when its calculation is complete, which allows the primary thread to block execution with WaitAll until all ten Fibonacci objects have calculated a result. The Main method then displays each Fibonacci result.

using System;
using System.Threading;

public class Fibonacci
    private int _n;
    private int _fibOfN;
    private ManualResetEvent _doneEvent;

    public int N { get { return _n; } }
    public int FibOfN { get { return _fibOfN; } }

    // Constructor. 
    public Fibonacci(int n, ManualResetEvent doneEvent)
        _n = n;
        _doneEvent = doneEvent;

    // Wrapper method for use with thread pool. 
    public void ThreadPoolCallback(Object threadContext)
        int threadIndex = (int)threadContext;
        Console.WriteLine("thread {0} started...", threadIndex);
        _fibOfN = Calculate(_n);
        Console.WriteLine("thread {0} result calculated...", threadIndex);

    // Recursive method that calculates the Nth Fibonacci number. 
    public int Calculate(int n)
        if (n <= 1)
            return n;

        return Calculate(n - 1) + Calculate(n - 2);

public class ThreadPoolExample
    static void Main()
        const int FibonacciCalculations = 10;

        // One event is used for each Fibonacci object.
        ManualResetEvent[] doneEvents = new ManualResetEvent[FibonacciCalculations];
        Fibonacci[] fibArray = new Fibonacci[FibonacciCalculations];
        Random r = new Random();

        // Configure and start threads using ThreadPool.
        Console.WriteLine("launching {0} tasks...", FibonacciCalculations);
        for (int i = 0; i < FibonacciCalculations; i++)
            doneEvents[i] = new ManualResetEvent(false);
            Fibonacci f = new Fibonacci(r.Next(20, 40), doneEvents[i]);
            fibArray[i] = f;
            ThreadPool.QueueUserWorkItem(f.ThreadPoolCallback, i);

        // Wait for all threads in pool to calculate.
        Console.WriteLine("All calculations are complete.");

        // Display the results. 
        for (int i= 0; i<FibonacciCalculations; i++)
            Fibonacci f = fibArray[i];
            Console.WriteLine("Fibonacci({0}) = {1}", f.N, f.FibOfN);

Following is an example of the output.

launching 10 tasks...
thread 0 started...
thread 1 started...
thread 1 result calculated...
thread 2 started...
thread 2 result calculated...
thread 3 started...
thread 3 result calculated...
thread 4 started...
thread 0 result calculated...
thread 5 started...
thread 5 result calculated...
thread 6 started...
thread 4 result calculated...
thread 7 started...
thread 6 result calculated...
thread 8 started...
thread 8 result calculated...
thread 9 started...
thread 9 result calculated...
thread 7 result calculated...
All calculations are complete.
Fibonacci(38) = 39088169
Fibonacci(29) = 514229
Fibonacci(25) = 75025
Fibonacci(22) = 17711
Fibonacci(38) = 39088169
Fibonacci(29) = 514229
Fibonacci(29) = 514229
Fibonacci(38) = 39088169
Fibonacci(21) = 10946
Fibonacci(27) = 196418
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