title: Why does the SelectedItem not display in combobox
body: <p>With the below binding in XAML, the selected item is not displayed in a combobox (specifically, the current type is not displayed for a site). We are using the caliburn micro framework and DevForce Ideablade. MySite is an Ideablade entity.  </p>
  
  <pre><code>&lt;ComboBox Grid.Column="3" Grid.Row="3" 
            ItemsSource="{Binding MySiteTypes}"
            DisplayMemberPath="description"
            SelectedItem="{Binding MySite.SiteType, Mode=TwoWay}"
            SelectedValuePath="description"/&gt;
  </code></pre>
  
  <p>Code in the ViewModel:</p>
  
  <pre><code>    private BindableCollection&lt;SiteType&gt; _mySiteTypes;
      public BindableCollection&lt;SiteType&gt; MySiteTypes
      {
          get { return _mySiteTypes; }
          set { _mySiteTypes = value; NotifyOfPropertyChange(() =&gt; MySiteTypes); }
      }
  </code></pre>
  
  <p>Once I set the site type with the combobox (the types are present in the dropdown), it correctly displays the type and changes the type in the database. What am I missing, why doesn't it want to display the existing type the first time?</p>