Mysterious `1 in XAML Datatype
http://msdn.microsoft.com/en-us/library/system.codedom.codetypereference.basetype.aspx
Generic types are formatted with the name of the type followed by a grave accent ("`") followed by a count of the generic type arguments.
So by removing the `1 you're actually saying the type is TreeCategoryand not TreeCategory<T>