Question

I am working on a file listener (for txt files) using FileSystemWatcher and want to append all new lines to a RichTextBox. Format new text to rich text needs alot of cpu power, so I decided to use a BackgroundWorker for this.

The program is still working, but form hangs when I lock MS Windows and relogin. Seems as BackgroundWorker thread is blocking the UI thread. There is no other effect of using BeginInvoke instead of Invoke for AppendText to RichTextBox.

When I pass the unformatted text to e.Result of bw_DoWork everything is working fine (no blocking threads). The temporary RichTextBox is not attached to the main form. How can I solve the “hang” problem?

Logic before code sample: FileSystemWatcher event Changed starts BackgroundWorker.RunWorkerAsync.

StringBuilder unformattedText;

private void bw_DoWork( object sender, DoWorkEventArgs e )
{
    // Write all new lines for this file to 
    // StringBuilder variable unformattedText
    ReadFromFile (...)

    // Need a temporary RichTextBox to create new formatted text
    RichTextBox tempRtb = new RichTextBox();

    // Split new unformatted text with regex, format some tokens 
    // and append each to temporary RichTextBox tempRtb
    HighlightWords( tempRtb, unformattedText );

    e.Result = tempRtb.Rtf;
}

private void bw_RunWorkerCompleted( object sender, RunWorkerCompletedEventArgs e )
{
    string rtfText = e.Result as string;

    // Append text to main RichTextBox 
    if (rtb.InvokeRequired)
    {
            rtb.BeginInvoke(
                new MethodInvoker( delegate
                {
                    AppendRichText( rtb, rtfText );
                } ) );
    }
    else
    { 
        AppendRichText( rtb, rtfText );
    }
}

private void AppendRichText(RichTextBox rtb, string text)
{
    // Append formatted text at the end of the RichTextBox
    rtb.Select( rtb.TextLength, 0 );
    rtb.SelectedRtf = text;

    rtb.ScrollToCaret();
}

Note: These are only the code parts to understand the problem. It’s not complete.

Answer 1

The RichTextBox cannot be created or manipulated on the worker thread. You MUST do it on the UI thread.

The bw_RunWorkerCompleted is run on the UI thread, so BeginInvoke is not required.

You need to move all the code after ReadFromFile into bw_RunWorkerCompleted and you will not need a temp RichTextBox anymore.