title: Get previous value of a cell in datagrid
body: <p>first of all sorry about my english. Im new with C#, wpf and this kind of things. 
  Im using entity framework and I'm binding a datagrid with an  ObservableCollection. 
  I need to get the old value of a cell and compare it with the new value.
  In database the entity has the old value because I haven't save the changes.
  My problem is that when I try to do a find (context.Find(reference.Id);), the element that returns the find has the new values of the grid. </p>
  
  <p>My XAML has:</p>
  
  <pre><code>&lt;DataGrid x:Name="datagrid" PreviewKeyDown="dataGrid_PreviewKeyDown" AutoGenerateColumns ="False" ItemsSource="{Binding Path=References, ElementName=referenceWindow, Mode=TwoWay}"
  </code></pre>
  
  <p>My .cs something like this:</p>
  
  <pre><code> ObservableCollection&lt;Reference&gt; References = new  ObservableCollection&lt;Reference&gt; References(context.References);
  </code></pre>
  
  <p>How can I get the old value?</p>
  
  <p>Thanks</p>