public form()

{
initializeComponent();
textbox.text.keypress + = new keypressEventHandler(text.box_keypress);
}

Now drag and down a textbox and click on "Keypress event"

private void textbox_keypress(obj sender, event e)

{
char pressedkey = e.keychar;
if(char.IsLetter(pressedkey)||char.IsSeparator(PressedKey)||char.IsPunction(pressedKey))
{
e.handled = false;
}
else
e.handled = true;
}