title: Setting combobox default value using SelectedIndex
body: <p>Why is that whenever I try to put SlectedIndex to 0, it always remains -1 ?</p>
  
  <pre><code>public partial class Window1 : Window
  {
      private ObservableCollection&lt;string&gt; _dropDownValues = new ObservableCollection&lt;string&gt;();
      public ObservableCollection&lt;string&gt; DropDownValues
      {
          get { return _dropDownValues; }
          set { _dropDownValues = value; }
      }
  
      private string _selectedValue;
      public string SelectedValue
      {
          get { return _selectedValue; }
          set { _selectedValue = value; }
      }
  
      public Window1()
      {
          InitializeComponent();
          DataContext = this;
  
          DropDownValues.Add("item1");
          DropDownValues.Add("item2");
          DropDownValues.Add("item3");
          DropDownValues.Add("item4");
          DropDownValues.Add("item5");
          DropDownValues.Add("item6");
  
          if (combotest.SelectedIndex == -1)
          {
              combotest.SelectedIndex = 0;
          }
      }
  }
  
  &lt;StackPanel HorizontalAlignment="Left" Margin="10"&gt;
          &lt;ComboBox Name="combotest"
              Margin="0 0 0 5"
              ItemsSource="{Binding DropDownValues}"
              SelectedValue="{Binding SelectedValue}"        
              Width="150"/&gt;     
      &lt;/StackPanel&gt;
  </code></pre>