title: How can i get original value if my databinding fails validation
body: <p>I have a working WPF dialog that uses DataGrid. The DataGrid is set to have <code>ItemsSource={Binding SomeCollection, Mode=TwoWay}</code>. This setup works fine, I can read values and update them from UI. <br/><br/>
  Later I have added Converters to validate values. I show blank when validation fails. Now I am interested in getting the original values back in case if validation fails. <br/><br/></p>
  
  <p>What options do i have here?</p>