title: using wpf datagridcomboboxcolumn's IsSynchronizedWithCurrentItem
body: <p>(see below for my own answer that I came up with after letting this percolate for days &amp; days)
  I am trying to achieve the following scenario in WPF.</p>
  
  <p>I have a datagrid that is displaying rows of data for viewing and additional data entry. It is a new app but there is legacy data.</p>
  
  <p>One particular field in the past has had data randomly entered into it. Now we want to limit that field's values to a particular list. So I'm using a DataGridComboBoxColumn. FWIW I have alternatively tried this with a DataGridTemplateColumn containing a ComboBox.</p>
  
  <p>At runtime, if the existing value is not on the list, I want it to display anyway. I just cannot seem to get that to happen. While I have attempted a vast array of solutions (all failures) here is the one that is most logical as a starting point.</p>
  
  <p>The list of values for the drop down are defined in a windows resource called "months".</p>
  
  <pre><code>&lt;DataGridComboBoxColumn x:Name="frequencyCombo"   MinWidth="100" Header="Frequency"
     ItemsSource="{Binding Source={StaticResource months}}"
     SelectedValueBinding="{Binding Path=Frequency,UpdateSourceTrigger=PropertyChanged}"&gt;
     &lt;DataGridComboBoxColumn.ElementStyle&gt;
       &lt;Style TargetType="ComboBox"&gt;
         &lt;Setter Property="IsSynchronizedWithCurrentItem" Value="False" /&gt;
       &lt;/Style&gt;
     &lt;/DataGridComboBoxColumn.ElementStyle&gt;
  &lt;/DataGridComboBoxColumn&gt;
  </code></pre>
  
  <p>What is happening is that if a value is not on the list then the display is blank. I have verified at runtime that the IsSynchronizedWithCurrentItem element is indeed False. It is just not doing what I am expecting.</p>
  
  <p>Perhaps I am just going down the wrong path here. Maybe I need to use a textbox in combination with the combobox. Maybe I need to write some code, not just XAML. I have spent hours trying different things and would be really appreciative of a solution. I have had a few suggestions to use this class or that control but without explanation of <em>how</em> to use it.</p>
  
  <p>Thanks a bunch!</p>