Developer .NET UPDATE, August 13, 2002

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(contributed by Bill Sheldon, [email protected])


  • Among software engineers, code reuse is a common practice. After engineers write a block of code that accomplishes a task, they don't want to have to write it again. One of the most reliable and effective ways to reuse code is through class inheritance. Unlike Visual Basic (VB), the Microsoft .NET Framework provides true inheritance. Because the .NET Framework is an object-based development environment, it provides not only interface inheritance (as in COM) but also implementation and source inheritance.

    The .NET Framework offers class libraries that are based on common functions. Unfortunately, the Visual Studio .NET development team made working with classes much easier for developers who use Visual C# .NET. Visual C# .NET's syntax is simpler than that of the other .NET languages. Plus, Visual Studio .NET includes a class wizard for Visual C# .NET only. One can only speculate as to why Microsoft included just the C# Class Wizard in Visual Studio .NET. Maybe the class wizards for the other languages weren't finished in time for Visual Studio .NET's release, which prompted Microsoft to bury the C# Class Wizard in Visual Studio .NET.

    If you create a Visual C# .NET class library project, the standard class opens within Visual Studio .NET's Solution Explorer. By switching from Solution Explorer to the Class View, you'll see your project's name followed by the default namespace. Right-clicking the project name in the Class View brings up a menu that contains an Add option. Selecting "Add a Class" on the Add option's submenu opens the C# Class Wizard. As I mentioned previously, Visual Studio .NET has a class wizard for Visual C# .NET only—if you create a Visual Basic .NET class library project and follow the same procedure, you won't get the same menu and submenu because they don't exist.

    The C# Class Wizard has a wizard-like interface that gives you a helpful framework with which to create classes and implement inheritance. I say "wizard-like" because when the wizard's dialog box opens, it has a Finish button but not a Next button. Suppose you want to create a new class named Class2 to go with the default Class1 that Visual Studio .NET automatically created when you began this project. After you specify the new class's name, you can use the list of options on the right to move to the next screen. Select Base Class and not Inheritance. You use the Inheritance dialog box to implement COM-style interface inheritance, which I'll discuss in a future commentary. You use the Base Class option to specify the base class from which your object will inherit an implemented set of base properties and methods. Inheritance from parent classes is part of implementation inheritance, in which both the source methods and properties are available without needing to be reimplemented in your class.

    The Base Class dialog box has only two edit fields, and both contain inaccurate information. The upper drop-down list displays the project's namespace. The lower drop-down list specifies the base class of the newly created class. By default, the wizard sets the base class to None. These settings are inaccurate because every .NET class inherits from System.Object and is part of the project namespace at a minimum. You can verify your object's inheritance by conducting the following exercise:

    1. Create a Windows forms project in Visual Basic .NET or Visual C# .NET and a reference to your sample library.
    2. Create an instance of your class, then call the ToString method. You'll see that the class and the method work. They work because unless you specify a different parent, every class inherits from System.Object. One of the beauties of the .NET Framework is that even if you select a different parent, that parent (or the class from which it inherits) winds up inheriting from System.Object at some point.
    3. Select a different parent in the class wizard by specifying Class1 as the base class for your new class, and click Finish.

    In Visual Studio .NET, you now have a new class. When the wizard finishes, Visual Studio .NET displays the code in the Code Editor (also called the Text Editor). Looking at the code, you'll find the line

    Public class Class2 : CSharpLibrarySample.Class1

    This statement tells Visual C# .NET that this class has a parent of Class1. If you want to achieve this same inheritance in a Visual Basic .NET project, you would use a class declaration such as

    Public Class Class2
       Inherits VBLibrarySample.Class1

    In Visual Basic .NET, you must put the Inherits keyword on a second line. If you try to put all the code on one line, Visual Basic .NET complains.

    What has this inheritance code really accomplished? To find out, switch to the code for Class1 in the Code Editor. In the class's code, edit the Class1 definition and have it inherit from System.Random.

    Having created this simple inheritance, you can now use a simple Windows form project to test your class. In the test project, create a variable called X as an instance of Class2. After you use the appropriate syntax to declare variable X, you can reference an object X method. Visual Studio .NET includes IntelliSense, a tool that displays language references. If you open IntelliSense's List Members option and type the letter X followed by a period, you'll get a list of all the methods and properties for that object. As this example shows, a class can have methods available, even if you haven't implemented any code for that class or its parent class.

    If you can't see the IntelliSense list of functions, select Options on the Tools menu. In the Options dialog box, select Text Editor. For the language you're using, make sure that the Auto List Members check box is selected and that the Hide Advanced Members check box is cleared. These settings will ensure that IntelliSense displays the list of available functions.

    The value of inheritance doesn't lie solely in the reuse of code. Inheritance also lets you manage complexity. For example, suppose you're creating an application that needs to access information in the registry. Instead of implementing System.Random, you can have a class that inherits from Microsoft.Win32.Registry. The Microsoft.Win32.Registry class can then act as a base class when you create other classes that need to access the registry. Because each new class would inherit from this base class, the functions to access the registry would be not only available but also already customized to read from the appropriate parent key and to handle errors in the same manner. Because you're using inheritance, you'll have only one copy of the registry-access code within the application. Every class that inherits from Microsoft.Win32.Registry reuses that code.

    There's much more to inheritance than what I've covered here. I'll continue covering object-related topics in the coming weeks. I'll show you how to develop class libraries that let you reuse code and discuss some of the other features of .NET's object-oriented nature. In the meantime, start looking into inheritance. It's a powerful and important part of the .NET Framework.


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    (contributed by Carolyn Mader, [email protected])


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