Vista - How to access the designer class for a WPF form

Hi

I have a WPF form with a class named X.xaml.cs and I need to inherit from
class Y. As C# doesn't support multiple inheritance of classes I cannot
inherit from Y because X is already inheriting from Window. I need to move
the inheritance of Window to Y then inherit Y by itself. I'm having
difficulties because when I specify to inherit from Y the compiler is
throwing an error "Partial class declarations must not specify different base
classes" and I'm assuming this is because the other part of the partial class
X is still inheriting from Window. How do I access the other partial class on
a WPF form? On a Windows Form I would just go to X.designer.cs. How do I
access the equivalent for a WPF form?

Thanks
Jason