CoClasses are defined by the interfaces they present to clients. While you can add any number of properties and methods to the implementation class of a CoClass, clients can only see those properties and methods that are exposed by interfaces associated with the CoClass. 

To associate an interface with a CoClass, right-click in the Implements page for the class and choose Insert Interface to display a list of interfaces from which you can choose. The list includes interfaces that are defined in the current type library and those defined in any type libraries that the current type library references. Choose an interface you want the class to implement. The interface is added to the page with its GUID and other attributes. 

If the CoClass was generated by a wizard, the Type Library Editor automatically updates the implementation class to include skeletal methods for the methods (including property access methods) of any interfaces you add this way.If you have the Apply Updates dialog enabled, the Type Library Editor notifies you before updating the sources and warns you of potential problems.