The ErrorProvider is built just for that: http://www.emoreau.com/Entries/Articles/2003/02/The-ErrorProvider-Control-(in-Windows-Forms).aspx


>I have a custom textbox class, and I would somehow like to add functionality to show when the textbox does not contain valid text. Basically, I just want a method that I can manually call that pops an exclamation point or other icon up next to the textbox.
>
>Anybody know how to do this? I know I can probably change the background color of the textbox or something, but that might look a little hokey.