C# multithreaded throbber form

Question

Working on a C# project which I would like to implement a "waiting" (throbber) indicator in a separate form. After much research and trial and error it appears as the suggested method of doing this is to load a form using a separate thread from the one from the current form/thread.

The reason I went with this method was because initially using the Show() method on the throbber form produced a transparent form. I cannot use ShowDialog because I need to run some code after the throbber is displayed, after which that completes I would like to close the throbber form.

Anyway .. after trying many different methods to load the throbber form in a separate thread I still get an error about trying to access it from a thread which is different from the one it was created in. Here is a skelton version of the project code that should shed some light on my issue:

the example I was working off of for multithreading was this popular link for creating your own spashscreen in a separate thread ... http://www.codeproject.com/Articles/5454/A-Pretty-Good-Splash-Screen-in-C

public class Main
{
    public void CheckData()
    {
        try
        {
            ProgressBar pb = new ProgressBar();
            pb.ShowProgressBar();
            //do data checking here
            pb.CloseForm()
        }   
        catch(Exception e)
        {
        }
   }
}

public partial class ProgressBar : Form
{
    static Thread ms_oThread = null;
    public bool shouldStop = false;
    static ProgressBar ms_ProgBar = null;
    public ProgressBar()
    {
        InitializeComponent();
        //DoWork();
    }

    public void ShowForm()
    {
        ms_ProgBar = new ProgressBar();
        Application.Run(ms_ProgBar);
    }
    public void CloseForm()
    {
        ms_ProgBar.Close();
    }

    public void ShowProgressBar()
    {
        // Make sure it is only launched once.
        if (ms_ProgBar != null)
            return;
        ms_oThread = new Thread(new ThreadStart(ShowForm));
        ms_oThread.IsBackground = true;
        ms_oThread.SetApartmentState(ApartmentState.STA);
        ms_oThread.Start();
        while (ms_ProgBar == null || ms_ProgBar.IsHandleCreated == false)
        {
            System.Threading.Thread.Sleep(1000);
        }
    }
}

Answer 1

You are creating your ProgressBar twice. Once in your main function, and once in your new thread. You are also calling your CloseWindow method from your main function (and on the window that is never shown), rather than on your new thread window.

You only want to create ProgressBar and show it using your new thread. Make your static ProgressBar field public so you can call close on it directly from Main, but make sure to use Invoke to do it since it's not on that Window's GUI thread.

Also, ShowProgressBar should be static.

Here's a rewrite attempt:

public class Main
{
    public void CheckData()
    {
        try
        {
            ProgressBar.ShowProgressBar();
            //do data checking here
            ProgressBar.CloseForm();
        }   
        catch(Exception e)
        {
        }
   }

}

public partial class ProgressBar : Form
{
    static ProgressBar _progressBarInstance;

    public ProgressBar()
    {
        InitializeComponent();
        //DoWork();
    }

    static void ShowForm()
    {
        _progressBarInstance = new ProgressBar();
        Application.Run(ms_ProgressBar);
    }

    public static void CloseForm()
    {
         _progressBarInstance.Invoke(new Action(_progressBarInstance.Close));
         _progressBarInstance= null;
    }

    public static void ShowProgressBar()
    {
        // Make sure it is only launched once.
        if (_progressBarInstance != null)
           return;

        var ms_oThread = new Thread(new ThreadStart(ShowForm));
        ms_oThread.IsBackground = true;
        ms_oThread.SetApartmentState(ApartmentState.STA);
        ms_oThread.Start();
    }
}