Wait for threads in a ThreadPool object to complete

Introduction

If you use the System.Threading.ThreadPool object, you know you can create many worker threads. The following code snippet waits until all worker threads have completed, or until the specified timeout.

Background

I use the System.Threading.ThreadPool extensively when creating Windows services. I usually spawn a worker thread for every service request. When the user attempts to pause or stop the service, I send a message to all threads to shut down, and then, in my main thread, must wait for all the threads to complete. If the service does not wait for all threads to complete, you get a nasty timeout error from Windows claiming the service 'did not respond'. I believe it is more professional to do the waiting in the service and not expose the user to the error dialog.

There are other ways to wait for all threads to complete, but none are as simple and as elegant as the code snippet below.

Using the code

I usually create a method called WaitForThreads() and define a timeout value (in seconds) as a property of my service class.

I call WaitForThreads() as the last action in my OnStop() and OnPause() methods. The timeout value is there just in case some worker thread misbehaves. An additional advantage to using a timeout is that I get to log all cases where the timeout was used - and fix the worker thread behavior.

/// <summary>
/// Blocks until all worker threads have returned to the thread pool.
/// A good timeout value is 30 seconds.
/// </summary>
protected void WaitForThreads()
{
    int    maxThreads   = 0;
    int    placeHolder  = 0;
    int    availThreads = 0;
    int    timeOutSeconds = YourTimeoutPropertyHere;

    //Now wait until all threads from the Threadpool have returned
    while (timeOutSeconds > 0)
    {
        //figure out what the max worker thread count it
        System.Threading.ThreadPool.GetMaxThreads(out 
                             maxThreads,out placeHolder);
        System.Threading.ThreadPool.GetAvailableThreads(out availThreads, 
                                                       out placeHolder);

        if (availThreads == maxThreads) break;
        // Sleep
        System.Threading.Thread.Sleep(TimeSpan.FromMilliseconds(1000));
        --timeOutSeconds;
    }
    // You can add logic here to log timeouts
}

Points of Interest

When creating services, make sure you can interrupt executing threads. It is very difficult to stop a service that has runaway threads. I create a separate class to be used by the worker threads and keep track of each thread in a master collection in my service class. Every worker thread class I have has an Interrupt() method which sets a flag causing the thread to exit.

You can use Thread.Abort to stop threads - but MSDN documentation recommends against it. In my experience, design your worker thread classes with reasonable checkpoints (preferably before starting any lengthy operations), and when you are waiting on a resource, make sure the service class can interrupt the wait.

History

Version 1.0, based on an idea suggested by Jeremy Thomas.