WPF

Karl Shifflett wrote:

You can add the extra row in the data in your business layer or data layer

Karl Shifflett wrote:

extend the combobox to function like the ASP.NET dropdown control that has this built in

public static void NullableDataSource<T>(this ComboBox source, IList<T> binding) 
  where T : class, new()
{
  if (binding != null)
  {
    binding.Insert(0, "<Null>");
  }
  source.DataSource = binding;
}

<UserControl.Resources>
    <!-- ToolTip Style -->
    <Style TargetType="{x:Type ToolTip}">
          <Setter Property="Background" Value="#FF000000" />
          <Setter Property="FontFamily" Value="Segoe UI" />
          <Setter Property="FontSize" Value="12" />
          <Setter Property="FontWeight" Value="Normal" />
          <Setter Property="Foreground" Value="#FFFFFFFF" />
          <Setter Property="Template">
                <Setter.Value>
                      <ControlTemplate TargetType="{x:Type ToolTip}">
                            <Border CornerRadius="4" Background="#FF000000" >
                                  <ContentPresenter />
                            </Border>
                      </ControlTemplate>
                </Setter.Value>
          </Setter>
    </Style>
</UserControl.Resources>