STA, MTA and OLE nightmare

Question

I have to include a .NET application into another .NET application as a plugin. The plugin interface requires me to inherit from a template form. The form is then attached in a MDI when the plugin is loaded.

Everything is working so far, but whenever I register for drag and drop events, set the autocomplete mode for a combobox or at various other situations I get the following exception:

...the current thread must be set to single thread apartment (STA) mode before OLE calls can be made. Ensure that your Main function has STAThreadAttribute marked on it...

The main application is running in MTA and developed by another company, so there is nothing I can do about it.

I tried to do the things that cause these exceptions in STA threads, but that didn't solve the problem either.

Has anyone been in the same situation? Is there anything I can do to solve the problem?

Answer 1

Update: The company released a new STA version. The question is no longer relevant.

Answer 2

You could try to spawn new thread and call CoInitialize with 0 on it (aparment threaded) and run your application in this thread. However you won't be to update controls directly within this thread you should use Control.Invoke for every UI modification.

I don't know if this is going to work for sure, but you could try it.

Answer 3

I recently ran into this problem myself while trying to read images from a web camera. What I ended up doing was creating a method that spawned a new STA thread, on which the single-thread method was run.

The problem

private void TimerTick(object sender, EventArgs e)
{
   // pause timer
   this.timer.Stop();

        try
        {
            // get next frame
            UnsafeNativeMethods.SendMessage(this.captureHandle, WindowsMessageCameraGetFrame, 0, 0);

            // copy frame to clipboard
            UnsafeNativeMethods.SendMessage(this.captureHandle, WindowsMessageCameraCopy, 0, 0);

            // notify event subscribers
            if (this.ImageChanged != null)
            {
                IDataObject imageData = Clipboard.GetDataObject();

                Image image = (Bitmap)imageData.GetData(System.Windows.Forms.DataFormats.Bitmap);

                this.ImageChanged(this, new WebCamEventArgs(image.GetThumbnailImage(this.width, this.height, null, System.IntPtr.Zero)));
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("Error capturing the video\r\n\n" + ex.Message);
            this.Stop();
        }
    }
   // restart timer
   Application.DoEvents();

   if (!this.isStopped)
   {
      this.timer.Start();
   }
}

The solution: Move the single-thread logic to its own method, and call this method from a new STA thread.

private void TimerTick(object sender, EventArgs e)
{
    // pause timer
    this.timer.Stop();

    // start a new thread because GetVideoCapture needs to be run in single thread mode
    Thread newThread = new Thread(new ThreadStart(this.GetVideoCapture));
    newThread.SetApartmentState(ApartmentState.STA);
    newThread.Start();

    // restart timer
    Application.DoEvents();

    if (!this.isStopped)
    {
        this.timer.Start();
    }
}

/// <summary>
/// Captures the next frame from the video feed.
/// This method needs to be run in single thread mode, because the use of the Clipboard (OLE) requires the STAThread attribute.
/// </summary>
private void GetVideoCapture()
{
    try
    {
        // get next frame
        UnsafeNativeMethods.SendMessage(this.captureHandle, WindowsMessageCameraGetFrame, 0, 0);

        // copy frame to clipboard
        UnsafeNativeMethods.SendMessage(this.captureHandle, WindowsMessageCameraCopy, 0, 0);

        // notify subscribers
        if (this.ImageChanged!= null)
        {
            IDataObject imageData = Clipboard.GetDataObject();

            Image image = (Bitmap)imageData.GetData(System.Windows.Forms.DataFormats.Bitmap);

            // raise the event
            this.ImageChanged(this, new WebCamEventArgs(image.GetThumbnailImage(this.width, this.height, null, System.IntPtr.Zero)));
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show("Error capturing video.\r\n\n" + ex.Message);
        this.Stop();
    }
}