Hi palraj001,
I think this may help you:
private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
if (e.KeyCode == Keys.F4 || (e.KeyCode == Keys.F4 && Control.ModifierKeys == Keys.Control) || (e.KeyCode == Keys.F4 && Control.ModifierKeys == Keys.Alt) || (e.KeyCode == Keys.F4 && Control.ModifierKeys == Keys.Control && Control.ModifierKeys == Keys.Alt))
{
}
else
{
}
}
The example above restricts F4 key in every combination with modifiers.
Happy Coding :)