Answer

It depends on where you create your element.
x:Key is only valid in the scope of a ResourceDictionary element. x:Key is used as the primary identifier for elements in the ResourceDictionary. Specifying x:Name is not considered valid (it will cause an exception when accessing resources).
On the other hand, x:Name is valid in the scope of everything but a ResourceDictionary. x:Key is not valid outside the ResourceDictionary scope.

Btw, lazy for update the blog due to my busy work ,enjoy~~hoho