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Inheriting Interfaces

It's no secret that I love interfaces (I did a whole column about them once). As you add more interfaces to your application you may find that you have several interfaces that look very much alike. These two, for example:

Public Interface ICustomer
    Property Id As Integer
    Property Name As String
    Sub Buy()
End Interface

Public Interface IVendor
    Property Id As Integer
    Property Name As String
    Sub Sell()
End Interface

But, looking at those interfaces, there's obviously the idea of a "business partner" buried in this interface design in the shared Id and Name property. It wouldn't be surprising to find that there are other interfaces in this application that share those Id and Name properties.

You can implement that "business partner" (and simplify maintenance of your application) by defining the business partner interface and then having the ICustomer and IVendor interfaces inherit from it. The result would look like this:

Public Interface IBusinessPartner
    Property Id As Integer
    Property Name As String
End Interface

Public Interface ICustomer
    Inherits IBusinessPartner
    Sub Buy()
End Interface

Public Interface IVendor
    Inherits IBusinessPartner
    Sub Sell()
End Interface

There are lots of benefits to building this inheritance structure: You can now extend both the IVendor and ICustomer interfaces with shared members by adding them to IBusinessPartner. If you ever need to add another "business partner" interface, all the common work is done for you: Your new interface just needs to inherit from IBusinessPartner.

Finally, a variable declared as IBusinessPartner will work with any class that implements ICustomer or IVendor, giving your application more flexibility.

Posted by Peter Vogel on 01/22/2015 at 2:20 PM


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