I wanted to use a common base class for a set of UserControls in my WPF application, but when I changed the class’s ancestor in the code file and compiled I would get the error

“Partial declarations of must not specify different base classes”

This is because when you compile a WPF application Visual Studio generates a partial class in a code-behind file automatically, the base type specified is always “UserControl”.  To solve this problem change your XAML from this

   1:  <UserControl

   2:    x:Class="MyApp.MyControl"

   3:  />

To this

   1:  <local:SomeBaseTypeYouWantToUse

   2:    x:Class="MyApp.MyControl"

   3:    xmlns:local="clr-namespace:NameSpace.To.Your.BaseClass"

   4:  />