title: How to accept just certain characters into a textbox?
body: <p>How do I limit textbox from accepting letter A,B,C,D only? I've tried this code, but it still accepts letters aside from letters A,B,C,D.</p>
  
  <pre><code>e.Handled = !char.IsDigit(e.KeyChar) &amp;&amp; !char.IsControl(e.KeyChar);
  </code></pre>