Question

I have an application wherein two threads write asynchronously to a single textbox. It works, except that the second thread to write to the textbox overwrites the line that the first thread just wrote. Any thoughts or insight into the problem would be greatly appreciated. I am using Microsoft Visual C# 2008 Express Edition. Thanks.

  delegate void SetTextCallback(string text);

  private void SetText(string text)
  {
     this.textBox1.Text += text;
     this.textBox1.Select(textBox1.Text.Length, 0);
     this.textBox1.ScrollToCaret();
  }

  private void backgroundWorkerRx_DoWork(object sender, DoWorkEventArgs e)
  {
     string sText = "";

     // Does some receive work and builds sText

     if (textBox1.InvokeRequired)
     {
        SetTextCallback d = new SetTextCallback(SetText);
        this.Invoke(d, new object[] { sText });
     }
     else
     {
        SetText(sText);
     }
  }

Answer 1

EDIT: This might not solve the problem, but you might want to handle the ProgressChanged event of the BackgroundWorkers and set the text there.

For example:

void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e) {
    SetText((string)e.UserState);
}  //Make this method handle the RunWorkerCompleted for both workers

//In DoWork:
    worker.ReportProgress(0, sText);

ProgressChanged is fired on the UI thread, so you don’t need to call Invoke.

By the way, you should probably rename SetText to AppendText to make the code clearer.
Also, you can use the built-in delegate Action<String> instead of making your own SetTextCallback delegate type.

EDIT: Also, you should probably move the InvokeRequired check to SetText.

For example:

private void AppendText(string text) {
    if(textBox1.InvokeRequired) {
        textBox1.Invoke(new Action<string>(AppendText), text);
        return;
    }
    this.textBox1.AppendText(text);
    this.textBox1.SelectionStart = textBox1.TextLength;
    this.textBox1.ScrollToCaret();
}