The Book of Visual Studio .NET - OOP in VB .NET Crash Course

Polymorphism

Abstraction is an interface implemented to represent a real-world object; encapsulation is the implementation of a black box through interface and implementation separation; and polymorphism is the ability to implement the interface of another class into multiple classes or to implement multiple interfaces on a single class. This method of implementation is referred to as interface-based programming. A vehicle is a good example of polymorphism. A vehicle interface would only have those properties and methods that all vehicles have, a few of which might include paint color, number of doors, accelerator, and ignition. These properties and methods would apply to all types of vehicles including cars, trucks, and semi-trucks.

Polymorphism will not implement code behind the vehicle's properties and methods. (That's the job of inheritance covered in the next section.) Instead, polymorphism is the implementation of an interface. If the car, truck, and semitruck all implement the same vehicle interface, then the client code for all three classes can be exactly the same.

Implementing the vehicle interface only requires the declaration of properties and methods. To create a new interface, use t he Interface keyword in place of the Class keyword. The client implementing the new interface can do so by using the Implements keyword as shown in the example:

Implements Ivehicle

After using the Implements keyword, you will notice that Intellisense displays the properties and methods of the IVehicle interface. Using the Implements keyword will only give access to the properties and methods of the IVehicle interface; however, you must provide your own code behind the methods and property declarations to match the interface.

Inheritance

Inheritance is the ability to apply another class's interface and code to your own class. Remember, with polymorphism, you got the interface; however, you must apply your own code. The power of inheritance is the ability to inherit code, saving developers time. This type of inheritance is called implementation inheritance. To inherit another class, use the Inherits keyword.

Visual inheritance is the ability to inherit another form's look and feel onto another. Remember, everything in .NET is a class, including forms. If you create a project that exists in the MyApp namespace, create a form name MyBaseForm. The following code will inherit the MyBaseForm within our new form:

Public Class MyNewForm
Inherits MyApp.MyBaseForm
End Class

Properties

Properties are part of a program's interface and describe the characteristics of a class. These properties hold information about a class or, when loaded into memory, an object. Properties, as they exist in classes, are often referred to as "data." When a reference is made to a class's data, you will know that the reference is actually directed toward a class's property.

To create a property, use t he Property keyword and t hen define the type of property you are implementing. Properties can be read-only, write-only, or read and writable. To define the characteristics of properties, use t he keywords ReadOnly, WriteOnly, or supply no definition at all to implement both read and write ability.

Visual Studio .NET makes properties easier to implement by adding the basic shell of property code based on the property's scope definition. Unlike Visual Basic, Visual Basic .NET automatically supplies code for both read and write functionality: "Get" for read ability method and "Set" for write access to a property.

Create a new class, type the following code, and press ENTER:

Public Property FName() As String

Visual Studio .NET will automatically fill in the rest of the code that is required by the FName property:

Get
Return m_FName
End Get
Set(ByVal Value As String)
m_FName = Value
End Set
End Property

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This article was originally published on February 3, 2003