最简单的方法来监视组合框值之间的变化

I'm looking for an elegant way to track changes between values for a combo box. What I'm looking to do is fire a custom event when the SelectionChanged event happens, but only for a specific value changes. This implies knowing what the initial value was. The event will only be fired when the initial value is changed from z. If the initial value is a, b, or c, the event will not be fired. But if the initial value was z, it will be fired.I'm looking for an elegant way to track changes