'<typename>' cannot be used as an attribute because it has 'MustOverride' methods that have not been overridden
Classes with MustOverride methods cannot be used as attributes.
MustOverride members of attribute classes can only be used from derived classes that override such members.
Error ID: BC31507
To correct this error
Remove the MustOverride modifier from attribute class members.
Implement MustOverride members in a derived class, and use that class as an attribute.