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Subclassing Multiple Classes

Class Member Compatibility

When you create a subclass derived from multiple classes, the subclass inherits the properties, methods, and events defined by all specified superclasses. If more than one superclass defines a property, method, or event having the same name, there must be an unambiguous resolution to the multiple definitions. You cannot derive a subclass from any two or more classes that define incompatible class members.

There are various situations where you can resolve name and definition conflicts, as described in the following sections.

Property Conflicts

If two or more superclasses define a property with the same name, then at least one of the following must be true:

  • All, or all but one of the properties must have their SetAccess and GetAccess attributes set to private

  • The properties have the same definition in all superclasses (for example, when all superclasses inherited the property from a common base class)

Method Conflicts

If two or more superclasses define methods with the same name, then at least one of the following must be true:

  • The method's Access attribute is private so only the defining superclass can access the method.

  • The method has the same definition in all subclasses. This situation can occur when all superclasses inherit the method from a common base class and none of the superclasses override the inherited definition.

  • The subclass redefines the method to disambiguate the multiple definitions across all superclasses. This means that the superclass methods must not have their Sealed attribute set to true.

  • Only one superclass defines the method as Sealed, in which case, the subclass adopts the sealed method definition.

  • The superclasses define the methods as Abstract and rely on the subclass to define the method.

Event Conflicts

If two or more superclasses define events with the same name, then at least one of the following must be true:

  • The event's ListenAccess and NotifyAccess attributes must be private.

  • The event has the same definition in all superclasses (for example, when all superclasses inherited the event from a common base class)

Using Multiple Inheritance

Resolving the potential conflicts involved when defining a subclass from multiple classes often reduces the value of this approach. For example, problems can arise when you enhance superclasses in future versions and introduce new conflicts.

Reduce potential problems by implementing only one unrestricted superclass. In all other superclasses, all methods are abstract and must be defined by a subclass or inherited from the unrestricted superclass.

In general, when using multiple inheritance, ensure that all superclasses remain free of conflicts in definition.

See Defining a Subclass for the syntax used to derive a subclass from multiple superclasses.

See Supporting Both Handle and Value Subclasses for techniques that provide greater flexibility when using multiple superclasses.

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