title: Don't allow editing a textbox once it was filled, allow only entering new values
body: <p>I have a mathematical calculation program.<br>
  I want to cycle between <code>TextBoxes</code> by pressing the tab button and it must select the value in the TextBox.<br>
  So I can use my program only for entering new values (I don't want to allow backspace to delete the current values).</p>
  
  <p>Currently, there is a blinking cursor at the right of the value.</p>
  
  <p>Note: I'm using Visual Studio 2013</p>