title: Input handling in WinForm
body: <p>What is the best way to block certain input keys from being used in a TextBox with out blocking special keystrokes such as <kbd>Ctrl</kbd>-<kbd>V</kbd>/<kbd>Ctrl</kbd>-<kbd>C</kbd>?</p>
  <p>For example, only allowing the user to enter a subset of characters or numerics such as  A or B or C and nothing else.</p>