title: WPF: Update cell in datagrid when another cell is selected
body: <p>We have a DataGrid composed of DataGridTemplateColumns.<br>
  Some of the columns contain simple TextBlocks, while others use ComboBoxes.  </p>
  
  <p>When the user clicks on a cell with a ComboBox, we need to update a TextBlock in the same row with the current value of the clicked-on ComboBox.<br>
  This is easy enough to do when a combobox selected value changes (the property bound to the combobox updates the property bound to the texblock when the combobox's value changes), but I can't figure out how to do it when the combobox cell is merely selected.<br>
  The SelectionUnit on the datagrid is CellOrRowHeader.</p>
  
  <p>I have been struggling trying to extract a value from DataGrid.CurrentCell, from within a SelectedCellsChangedEvent handler, etc., with no success.<br>
  Why is it so difficult to get the current value of a datagrid cell when it is selected?!<br>
  Any pointers would be appreciated...</p>