//create a delegate
delegate void AddToTextBoxDelegate(ref TextBox textBox1, string text);

//create a function which the delegate will use
private void AddToTextBox(ref TextBox textBox1, string text)
{
    textBox1.Text += text;
}

//use the delegate
name_of_your_textbox.Invoke(new AddtoTextBoxDelegate(UpdateTextBox), new object[] { ref name_of_your_textbox, string_to_write_into_your_textbox });