title: How to use the previous value when validating a WPF DataGrid cell entry?
body: <p>I have a WPF <code>DataGrid</code>.  I would like to restrict the values a user may enter in a particular column, such as the following:</p>
  
  <p>Column A values may only increase (new value > old value).</p>
  
  <p>Column B values may only decrease (new value &lt; old value).</p>
  
  <p>If the user-entered value is invalid, I want to revert to the previous valid value.</p>
  
  <p>I thought that I might be able to do this in a custom <code>ValidationRule</code>, but I don't see a way to access the previous value of the cell from the <code>Validate()</code> function.</p>
  
  <p>I would appreciate any help!</p>