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.

For more information about binding and dependency property, you could refer to the following links.

http://msdn2.microsoft.com/en-us/library/ms753192.aspx

http://msdn2.microsoft.com/en-us/library/ms750612.aspx