update text box from another thread

3.cs

delegate void SetTextCallback(string newText);
private void add_output(string text)
{
    if (this.output_textBox.InvokeRequired)
    {
        SetTextCallback d = new SetTextCallback(add_output);
        this.Invoke(d, new object[] { text });
    }
    else
    {
        this.output_textBox.Text += text;
    }
}