title: Set textbox text without removing binding
body: <p>I have something like:</p>
  
  <pre><code>&lt;TextBox Text={Binding MyDate, 
                 ValidatesOnNotifyDataErrors=True, 
                 ValidatesOnDataErrors=True} /&gt;
  </code></pre>
  
  <p><strong>I have a checkbox than when checked need to make the textbox readonly and display "N/A"</strong>. Validations are already created for <code>MyDate</code> and if a validation error triggers it will display an error message on the textbox. If I bind that textbox to a different property of type string then I will have to create validations for that string. <strong>How can I keep the textbox binded to MyDate and display N/A?</strong></p>