MSIR.0874 Class (classname) contains unimplemented abstract methods

An extension class of an abstract class did not declare methods that Implement all abstract methods in the base class. If the extension class is, itself, abstract, it must still Implement all abstract methods in the base class, possibly as abstract. This problem could also arise if an abstract method is added to an abstract class after it's been extended. For this reason, it is recommended that, instead of adding abstract methods to extended classes, you use overridable methods wherever possible.