title: How do I add a default comboboxitem "--Select user--"
body: <p>I am developing a WPF application using MVVM pattern, I have a combobox with itemssource bounded from viewmodel, I want to have a Default option in the combobox like "--Select user--", what is the best approach to do this.
  This is my XAML code:</p>
  
  <pre><code>&lt;ComboBox Grid.Column="1" HorizontalAlignment="Left" Margin="5.2,8.2,0,7.8" Grid.Row="5" Width="340" ItemsSource="{Binding Path=Users}" IsSynchronizedWithCurrentItem="True" SelectedItem="{Binding SelectedUser}" Grid.ColumnSpan="2"&gt;
              &lt;ComboBox.ItemTemplate&gt;
                  &lt;DataTemplate&gt;
                      &lt;TextBlock&gt;
                          &lt;TextBlock.Text&gt;
                              &lt;MultiBinding StringFormat="{}{0} {1}"&gt;
                                  &lt;Binding Path="FirstName"/&gt;
                                  &lt;Binding Path="LastName"/&gt;
                              &lt;/MultiBinding&gt;
                          &lt;/TextBlock.Text&gt;
                      &lt;/TextBlock&gt;
                  &lt;/DataTemplate&gt;
              &lt;/ComboBox.ItemTemplate&gt;
  &lt;/ComboBox&gt;
  </code></pre>