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