Worker Threads in C#

• Download demo project - 8 Kb

Introduction

The .NET framework provides a lot of ways to implement multithreading programs. I want to show how we can run a worker thread which makes syncronous calls to a user interface (for example, a thread that reads a long recordset and fills some control in the form).

To run thread I use:

• Thread instance and main thread function
  
• Two events used to stop thread. First event is set when main thread wants to stop worker thread; second event is set by worker thread when it really stops.

.NET allows you to call System.Windows.Forms.Control functions only from the thread in which the control was created. To run them from another thread we need to use the Control.Invoke (synchronous call) or Control.BeginInvoke (asynchronous call) functions. For tasks like showing database records we need Invoke.

To implement this we will use:

• A Delegate type for calling the form function. Delegate instance and function called using this delegate
  
• The Invoke call from the worker thread.

The next problem is to stop the worker thread correctly. The steps to do this are:

• Set the event "Stop Thread"
• Wait for the event "Thread is stopped"
• Wait for the event process messages using the Application.DoEvents function. This prevents deadlocks because the worker thread makes Invoke calls which are processed in the main thread.

The thread function checks every iteration whether the "Stop Thread" event has been set. If the event is set the function invokes clean-up operations, sets the event "Thread is stopped" and returns.

Demo project has two classes: MainForm and LongProcess. The LongProcess.Run function runs in a thread and fills the list box with some lines. The worker thread may finish naturally or may be stopped when user presses the "Stop Thread" button or closes the form.

Code fragments

// MainForm.cs

namespace WorkerThread
{
    // delegates used to call MainForm functions from worker thread
    public delegate void DelegateAddString(String s);
    public delegate void DelegateThreadFinished();

    public class MainForm : System.Windows.Forms.Form
    {
        // ...

        // worker thread
        Thread m_WorkerThread;

        // events used to stop worker thread
        ManualResetEvent m_EventStopThread;
        ManualResetEvent m_EventThreadStopped;

        // Delegate instances used to call user interface functions 
        // from worker thread:
        public DelegateAddString m_DelegateAddString;
        public DelegateThreadFinished m_DelegateThreadFinished;

        // ...

        public MainForm()
        {
            InitializeComponent();

            // initialize delegates
            m_DelegateAddString = new DelegateAddString(this.AddString);
            m_DelegateThreadFinished = new DelegateThreadFinished(this.ThreadFinished);

            // initialize events
            m_EventStopThread = new ManualResetEvent(false);
            m_EventThreadStopped = new ManualResetEvent(false);

        }

        // ...

        // Start thread button is pressed
        private void btnStartThread_Click(object sender, System.EventArgs e)
        {
            // ...
            
            // reset events
            m_EventStopThread.Reset();
            m_EventThreadStopped.Reset();

            // create worker thread instance
            m_WorkerThread = new Thread(new ThreadStart(this.WorkerThreadFunction));

            m_WorkerThread.Name = "Worker Thread Sample";   // looks nice in Output window

            m_WorkerThread.Start();

        }


        // Worker thread function.
        // Called indirectly from btnStartThread_Click
        private void WorkerThreadFunction()
        {
            LongProcess longProcess;

            longProcess = new LongProcess(m_EventStopThread, m_EventThreadStopped, this);

            longProcess.Run();
        }

        // Stop worker thread if it is running.
        // Called when user presses Stop button or form is closed.
        private void StopThread()
        {
            if ( m_WorkerThread != null  &&  m_WorkerThread.IsAlive )  // thread is active
            {
                // set event "Stop"
                m_EventStopThread.Set();

                // wait when thread  will stop or finish
                while (m_WorkerThread.IsAlive)
                {
                    // We cannot use here infinite wait because our thread
                    // makes syncronous calls to main form, this will cause deadlock.
                    // Instead of this we wait for event some appropriate time
                    // (and by the way give time to worker thread) and
                    // process events. These events may contain Invoke calls.
                    if ( WaitHandle.WaitAll(
                        (new ManualResetEvent[] {m_EventThreadStopped}), 
                        100,
                        true) )
                    {
                        break;
                    }

                    Application.DoEvents();
                }
            }
        }

        // Add string to list box.
        // Called from worker thread using delegate and Control.Invoke
        private void AddString(String s)
        {
            listBox1.Items.Add(s);
        }

        // Set initial state of controls.
        // Called from worker thread using delegate and Control.Invoke
        private void ThreadFinished()
        {
            btnStartThread.Enabled = true;
            btnStopThread.Enabled = false;
        }

    }
}

// LongProcess.cs

namespace WorkerThread
{
    public class LongProcess
    {
        // ...
    
        // Function runs in worker thread and emulates long process.
        public void Run()
        {
            int i;
            String s;

            for (i = 1; i <= 10; i++)
            {
                // make step
                s = "Step number " + i.ToString() + " executed";

                Thread.Sleep(400);

                // Make synchronous call to main form.
                // MainForm.AddString function runs in main thread.
                // (To make asynchronous call use BeginInvoke)
                m_form.Invoke(m_form.m_DelegateAddString, new Object[] {s});


                // check if thread is cancelled
                if ( m_EventStop.WaitOne(0, true) )
                {
                    // clean-up operations may be placed here
                    // ...

                    // inform main thread that this thread stopped
                    m_EventStopped.Set();

                    return;
                }
            }

            // Make synchronous call to main form
            // to inform it that thread finished
            m_form.Invoke(m_form.m_DelegateThreadFinished, null);
        }

    }
}