title: How to make a textBox accept only Numbers and just one decimal point in Windows 8
body: <p>I am new to windows 8 phone. I am writing a calculator app that can only accept numbers in the textbox and just a single decimal point. how do I prevent users from inputting two or more decimal Points in the text box as the calculator cant handle that.</p>
  
  <p>I have been using Keydown Event, is that the best or should I use Key up?</p>
  
  <pre><code>private void textbox_KeyDown(object sender, System.Windows.Input.KeyEventArgs e) {
  
  }
  </code></pre>