Do you mean that you want to bind a variable (not a property) to ComboBox.SelectedIndex? If so, I think that the better method to do this is bind a property to ComboBox.SelectedItem. The following example shows how to bind a property to the ComboBox.SelectedItem. If you want to have more functionality from the binding, you should have the property is a dependency property.
Code Snippet
<Window x:Class="LearningSolution.MainWindow"xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"xmlns:system="clr-namespace:System;assembly=mscorlib"xmlns:local="clr-namespace:LearningSolution"Name="TheWindow"><Window.Resources><ObjectDataProvider x:Key="EnumValues"MethodName="GetValues"ObjectType="{x:Type system:Enum}"><ObjectDataProvider.MethodParameters><x:Type TypeName="local:LevelEnum" /></ObjectDataProvider.MethodParameters></ObjectDataProvider><DataTemplate x:Key="EnumValueTemplate"><TextBlock Text="{Binding}"/></DataTemplate></Window.Resources><StackPanel><ComboBox ItemsSource="{Binding Source={StaticResource EnumValues}}"ItemTemplate="{StaticResource EnumValueTemplate}"SelectedItem="{Binding ElementName=TheWindow, Path=TheLevel, Mode=OneWayToSource}"/><Button Click="Button_Click">Watch</Button></StackPanel></Window>public partial class MainWindow : Window{public LevelEnum TheLevel { get; set; }private void Button_Click(object sender, RoutedEventArgs e){MessageBox.Show(this.TheLevel.ToString());}}public enum LevelEnum { None = 0, First = 1, Second = 2, Third = 3 }
For more information about binding and dependency property, you could refer to the following links.