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Worker Threads in C#

, 30 Jul 2001 CPOL 745.9K 11.1K 299
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Running worker threads with syncronous calls to Windows controls


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()

            // 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

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

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



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

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


        // 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"

                // 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}), 
                        true) )


        // Add string to list box.
        // Called from worker thread using delegate and Control.Invoke
        private void AddString(String 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";


                // 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


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



This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

GeneralMy vote of 5 Pin
Somnath_dutta21-Aug-13 2:12
memberSomnath_dutta21-Aug-13 2:12 
GeneralMy vote of 5 Pin
Franc Morales16-Aug-13 20:20
memberFranc Morales16-Aug-13 20:20 
GeneralMy vote of 5 Pin
davemaster9923-Jun-13 20:38
memberdavemaster9923-Jun-13 20:38 
GeneralMy vote of 5 Pin
AU9-May-12 13:46
memberAU9-May-12 13:46 
GeneralMy vote of 4 Pin
Ali Fakoor7-Oct-11 23:36
memberAli Fakoor7-Oct-11 23:36 
GeneralJust making AddString() threadsafe Pin
qduda16-May-11 21:06
memberqduda16-May-11 21:06 
QuestionGood Template but Needs Example Pin
Fl0wmastr23-Mar-11 4:04
memberFl0wmastr23-Mar-11 4:04 
AnswerRe: Good Template but Needs Example Pin
Alex Fr23-Mar-11 8:00
memberAlex Fr23-Mar-11 8:00 
GeneralMy vote of 3 Pin
Fl0wmastr23-Mar-11 4:01
memberFl0wmastr23-Mar-11 4:01 
Generalmy vote of 5 Pin
Member 330244816-Feb-11 12:11
memberMember 330244816-Feb-11 12:11 
GeneralMy vote of 5 Pin
roland_h11-Jan-11 14:07
memberroland_h11-Jan-11 14:07 
GeneralСпасибо брателло!!! Pin
AlexForex15-Feb-10 1:12
memberAlexForex15-Feb-10 1:12 
GeneralRe: Спасибо брателло!!! Pin
Alex Fr15-Feb-10 8:08
memberAlex Fr15-Feb-10 8:08 
GeneralSir you totally rock! Pin
anshulskywalker8-May-09 6:55
memberanshulskywalker8-May-09 6:55 
GeneralEncapsulated Worker Thread and Threaded Queue Classes Pin
jakolito18-Mar-09 8:16
memberjakolito18-Mar-09 8:16 
GeneralThanks for this wonderful Article. Pin
KSuthar10-Dec-08 11:31
memberKSuthar10-Dec-08 11:31 
GeneralProblem with threads Pin
csjenci25-Aug-07 6:00
membercsjenci25-Aug-07 6:00 
GeneralThanks for the awesome article and requesting for Questions further. Pin
AmarjeetSinghMatharu28-Jul-07 3:45
memberAmarjeetSinghMatharu28-Jul-07 3:45 
GeneralRe: Thanks for the awesome article and requesting for Questions further. Pin
Alex Fr28-Jul-07 6:36
memberAlex Fr28-Jul-07 6:36 
GeneralRe: Thanks for the awesome article and requesting for Questions further. Pin
AmarjeetSinghMatharu28-Jul-07 22:20
memberAmarjeetSinghMatharu28-Jul-07 22:20 
Questionm_form and overload error Pin
Ashley Sanders20-Apr-07 11:21
memberAshley Sanders20-Apr-07 11:21 
GeneralRe: m_form and overload error Pin
Ashley Sanders23-Apr-07 5:35
memberAshley Sanders23-Apr-07 5:35 
Doh. I finally actually downloaded the demo and added the LongProcess function and the three members (m_XXX) to the LongProcess class.

Especially since this is marked for beginners, it would be helpful I think if these were not left out of the sample code (by all means hiding all the windows form junk is a good idea).

Otherwise this was such a great article.

I am just not a down loader by nature (I just want to go straight to modifying my own code). I guess if I had done that first, I'd have had no problem, but those few elements in the code sample would make it completely usable without the download.

GeneralRe: m_form and overload error Pin
mrloki2-Mar-08 10:47
membermrloki2-Mar-08 10:47 
GeneralVery usefull Pin
Dark-Balron3-Apr-07 1:21
memberDark-Balron3-Apr-07 1:21 
GeneralGreat Article Pin
steve_randomno219-Jan-07 5:35
membersteve_randomno219-Jan-07 5:35 

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