Why does this code only work in one instance?

Question

I've set my form's KeyPreview property to true.

I've added this code:

private void PlatypusScheduleForm_KeyDown(object sender, KeyEventArgs e) 
{
  if (e.KeyCode == Keys.F9)
  {
    tabControlPlatypi.SelectedTab = tabPageDuckBill;
  }
  else if (e.KeyCode == Keys.F10)
  {
    tabControlPlatypi.SelectedTab = tabPagePlatypus;
  }
}

When I mash F10, it works as expected; mashing F9, however, does nothing.

tabPageDuckBill is the design-time/default tabPage that displays. Why would F10 work in taking me to the "other" tab page, but F9 then not go back to the original?

Do you need to mash F10 to get it to work? When you set a breakpoint in the method, does F9 cause it to fire? — John Arlen, Aug 30 '12 at 17:05
Stepping through it when putting a breakpoint on the first line, it works; "live" (without the breakpoint), it doesn't...??? There is no reference in code to F9 anywhere else. — B. Clay Shannon-B. Crow Raven, Aug 30 '12 at 17:14
I've found the problem: I've got code in the TabControl's SelectedIndexChanged() event, which is messing things up. — B. Clay Shannon-B. Crow Raven, Aug 30 '12 at 17:21
..and yet I'm not SETTING the selectedIndex there, so it still seems macabre... — B. Clay Shannon-B. Crow Raven, Aug 30 '12 at 17:32

score 1 · Answer 1 · edited Aug 30 '12 at 17:40

I ran into this same problem in the past, and the problem persisted even after removing suspect code from the SelectedIndexChanged() event. I then used a different techniques that worked much better. Instead of using the form KeyDown event, I overrode the form ProcessCmdKey event as follow:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
  if (keyData == Keys.F9)
  {
    this.tabControl1.SelectedTab = tabPage1;
    return true;    
  }
  else if (keyData == Keys.F10)
  {
    this.tabControl1.SelectedTab = tabPage2;
    return true;
  }
  return base.ProcessCmdKey(ref msg, keyData);
}

score 1 · Accepted Answer · edited Aug 30 '12 at 17:43

I found that if I just did this:

private void Form1_KeyDown(object sender, KeyEventArgs e)
{
  if (e.KeyCode == Keys.F9)
  {
      tabControl1.SelectedTab = tabPage1;
      e.SuppressKeyPress = true;
  }
  else if (e.KeyCode == Keys.F10)
  {
      tabControl1.SelectedTab = tabPage2;
      e.SuppressKeyPress = true;
  }
}

it'll toggle back and forth just fine. Without that e.SuppressKeyPress = true;, however, it exhibited the behavior you mentioned.

Why does this code only work in one instance?

2 Answers2