title: WPF ComboBox with IsEditable="True" - How can I indicate that no match was found?
body: <p>Using the following simple text box as an example:</p>
  
  <pre><code>&lt;ComboBox IsEditable="True" SelectedItem="{Binding}"&gt;
      &lt;ComboBoxItem&gt;Angus/ComboBoxItem&gt;
      &lt;ComboBoxItem&gt;Jane&lt;/ComboBoxItem&gt;
      &lt;ComboBoxItem&gt;Steve&lt;/ComboBoxItem&gt;
  &lt;/ComboBox&gt;
  </code></pre>
  
  <p>I would like to allow the user to find their selection by typing in a name, so I have set <em>IsEditable</em> equal to true. Acceptable values for the property bound to <em>SelectedItem</em> are any one of the options in the list, or no selection (<em>null</em>). The problem is, there is no error indication by default in the event someone types in a name that isn't in the list.</p>
  
  <p>For example: a user could type "Bob", causing the <em>SelectedItem</em> property to be <em>null</em>, but not realize that Bob doesn't exist in the list. Instead I would like to provide a visual indication as soon as the ComboBox's <em>Text</em> property is not <em>null</em> or empty AND <em>SelectedItem</em> is <em>null</em>, and stop them from typing any more?</p>
  
  <p>My initial thought was a custom validation rule, but I don't know how to access both the Text and SelectedItem properties of the combobox.</p>