How to set focus to a control after validation in .NET

Question

I have a WinForm application with several input controls on a form. In the validation event handler (either Validating or Validated), I need to det

Answer 1

Yes, changing the focus during a Validating event is quite troublesome. The event is raised at the exact time the focus changes. The next control has already obtained the focus as far as Windows is concerned but the logical form state still has the focus at the control being validated. When you set e.Cancel to true, Winforms must undo the Windows focus state. When you don't, it must update the logical state after the event. There are a wide variety of things that can go wrong when you change focus yourself.

It is important that you wait until the focus has been sorted out. Best thing to do is to delay your code until everything is done running and the form goes idle again. You can cleanly do so by using the Control.BeginInvoke() method. Something like this:

    private delegate void ChangeFocusDelegate(Control ctl);

    private void textBox1_Validating(object sender, CancelEventArgs e) {
        int value;
        if (!int.TryParse(textBox1.Text, out value)) e.Cancel = true;
        else {
            if (value == 1) this.BeginInvoke(new ChangeFocusDelegate(changeFocus), textBox2);
            else this.BeginInvoke(new ChangeFocusDelegate(changeFocus), textBox3);
        }
    }
    private void changeFocus(Control ctl) {
        ctl.Focus();
    }