"Could not create an instance of type"

This is an interesting error message. Imagine you have been biulding up a wpf application using lots of user controls and databinding. Copying code from one control to another to save time on the typing. Then it is time to use 2 user controls together in the same window and then you get the error "Could not create an instance of type". Looking at this closely it happens during the initialization of one of the controls. Looking on the internet shows some articles about the problems of making wpf controls inherit from one another. It turned out to be much simpler. I only had forgotten to modify the depency object to be of the same type of the control.