Question

I have an application that holds an image in a picture box. When you press ctrl+C it copies the image to clipboard. I use a thread to perform the actual clipboard operations.

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (keyData == (Keys.Control | Keys.C))
    {
        clipboardThread = new Thread(copy_to_clipboard);
        clipboardThread.SetApartmentState(ApartmentState.STA);
        clipboardThread.Start();
        return true;
    }
    return base.ProcessCmdKey(ref msg, keyData);
}

private void copy_to_clipboard()
{
    if (pic_display.Image != null)
    {
        using (MemoryStream stream = new MemoryStream())
        {
            clipboardStatus.Text = "Copying image to clipboard...";
            pic_display.Image.Save(stream, ImageFormat.Png);
            var data = new DataObject("PNG", stream);
            Clipboard.Clear();
            Clipboard.SetDataObject(data, true);
            clipboardStatus.Text = "Copied successfully!";
        }
    }
}

Right now, if I repeatly send ctrl+C (eg: hold the keys down), a new thread would be spawned. How can I change the code so that I re-use the clipboard thread, and have it tell me whether it’s currently copying data or not so that I don’t try to execute another copy command while it’s still working.

UPDATE

Now it works

I’ve changed it to use a background worker

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (keyData == (Keys.Control | Keys.C))
    {
        if (!backgroundWorker1.IsBusy)
            backgroundWorker1.RunWorkerAsync();
        return true;
    }
    return base.ProcessCmdKey(ref msg, keyData);
}

private void copy_to_clipboard()
{
    using (var stream = new MemoryStream())
    {
        clipboardStatus.Text = "Copying image to clipboard...";
        pic_display.Invoke((Action)(() => {
            if (pic_display.Image != null)
              pic_display.Image.Save(stream, ImageFormat.Png); 
        }));
        if (stream.Position == 0) return; // No image was saved
        var data = new DataObject("PNG", stream);
        BeginInvoke ( (Action) ( ()=> {
            Clipboard.Clear();
            Clipboard.SetDataObject(data, true);
        }
        clipboardStatus.Text = "Copied successfully!";
    }
}

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    copy_to_clipboard();
}

But now an exception occurs on

Clipboard.Clear();

saying

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.

Answer 1

First off, you should be invoking when accessing UI controls from a background thread:

private void copy_to_clipboard()
{
    using (var stream = new MemoryStream())
    {
        clipboardStatus.Text = "Copying image to clipboard...";
        pic_display.Invoke((Action)()=> { 
            if (pic_display.Image != null)
              pic_display.Image.Save(stream, ImageFormat.Png); 
        });
        if (stream.Position == 0) return; // No image was saved
        var data = new DataObject("PNG", stream);
        Clipboard.Clear();
        Clipboard.SetDataObject(data, true);
        clipboardStatus.Text = "Copied successfully!";
    }
}

Then invoke that from a BackgroundWorker. Lots of examples of that are available on the web.