COM+ Programming with Visual Basic

Polymorphism I (Multiple Components—Single Interface)

The term polymorphism refers to many different things in object-oriented lingo. One aspect of polymorphism is that two classes may share a single interface but implement the methods in the interface slightly differently. To illustrate this meaning of polymorphism, let’s suppose we add a CSavings class to the project:

' Class CSavings

Option Explicit

Implements IAccount

Private m_cBalance As Currency
Private m_dInterest As Double

Private Property Get IAccount_Balance(  ) As Currency
    IAccount_Balance = m_cBalance 
End Property

Private Sub IAccount_MakeDeposit(ByVal Amount As Currency)
    m_cBalance = m_cBalance + Amount + (Amount * m_dInterest)
End Sub

Notice that the preceding code is very similar to that of the CChecking class, except that the MakeDeposit subroutine actually adds some interest to the Amount sent it (this is a very good savings account). This shows that the definition of the interface tells the implementer nothing about how the code for the function is to be written. Each class may have a different interpretation of the MakeDeposit routine. (Later, you will see that this is especially good for polymorphism.) The only requirement is that they have similar semantics—in other words, that the two implementations do roughly the same thing with slight variations; MakeDeposit in CSavings does not remove money from an account, for example.

At this point, our project has two implementation classes, CChecking and CSavings, and ...