If your button is a straight Command, you could have multiple instances of it. So maintaining a reference to the command on an extension would not work. The extension will always be a unique instance (by definition), so you can keep a reference to the extension on your two controls and use it to pass values back and forth (this is a model I use a lot.) You can also place a button on your toolcontrol along side of your combobox, that works nicely some times.