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Language: Visual Basic
Expertise: Intermediate

May 18, 1999

VB5 provides interface inheritance through the use of the Implements keyword. For example, CFullTimeEmployee can implement the IEmployee interface. This interface might include basic information such as name, social security number, and date of birth. Another class, CPartTimeEmployee can also implement the IEmployee interface. You can then write code against the IEmployee interface without regard to the type of employee. To supply additional functionality, you might create an IEmp2 interface. To test whether an object is of a certain type, use the TypeOf keyword at run time. The format is "TypeOf object Is class/interface". Here's how to define two classes:

 
Class CFullTimeEmployee:
Implements IEmployee
Implements IEmp2

Class CPartTimeEmployee
Implements IEmployee

Dim objMyFTE as New CFullTimeEmployee
Dim objMyPTE as New CPartTimeEmployee

Using TypeOf, you can query at run time which interfaces these objects support:

 
Query	Return
TypeOf objMyFTE Is CFullTimeEmployee	True
TypeOf objMyFTE Is IEmployee	True
TypeOf objMyFTE Is IEmp2	True
TypeOf objMyFTE Is CPartTimeEmployee	False
TypeOf objMyFTE Is Object	True
TypeOf objMyFTE Is IUnknown	True

TypeOf objMyPTE Is CPartTimeEmployee	True
TypeOf objMyPTE Is IEmployee	True
TypeOf objMyPTE Is IEmp2	False
TypeOf objMyPTE Is Object	True
TypeOf objMyPTE Is IUnknown	True
TypeOf objMyPTE Is CFullTimeEmployee	False