Partial declarations must not specify different base classes

I have a wpf page named StandardsDefault. In the code behind, StandardsDefault is inheriting Page, like all other pages.

<Page x:Class="namespace.StandardsDefault"

public partial class StandardsDefault : Page

Now I have created a new class CountryStandards which is inheriting StandardsDefault instead of page.

<Page x:Class="namespace.CountryStandards"

public partial class CountryStandards : StandardsDefault

I have not changed the XAML . I am getting the error as

"Partial declarations of 'CountryStandards' must not specify different base classes"

I think the problem may be that the designer is not inheriting the same class. But I need to somehow implement inheritance since there are many common methods which are to be used in many standard pages like CountryStandards

Can anyone help me out?