e.Gender = Gender.Male in .NET

Print Quick Response Code in .NET e.Gender = Gender.Male

A better way to anticipate the effect of inheritance is to pretend that all the nonover ridden routines in the base class have been pasted inside the derived class. So if they reference another property or method, they call the version of the member that s defined in the derived class not the original one defined in the base class. However, sometimes you want a piece of code in the base class to use the nonoverrid den version of the properties and methods it references. Let s use another example to clarify this concept. Let s say that a person can vote only if he or she is 18 years old, so the Person3 class contains this code:

Class Person3 Public BirthDate As Date Age is defined as the number of whole years passed from BirthDate. Overridable ReadOnly Property Age() As Integer Get

5:

Age = Now.Year BirthDate.Year If Now.DayOfYear < BirthDate.DayOfYear Then Age -= 1 End Get End Property ReadOnly Property CanVote() As Boolean Get Return (Age >= 18) End Get End Property End Class

The Employee3 class uses a looser definition of the age concept and overrides the Age property with a simpler version that returns the difference between the current year and the year when the employee was born:

Do you see the problem The CanVote property of an Employee3 object is inherited as is from the Person3 class but incorrectly uses the Age property defined in the Employee3 class rather than the original version in the base class. To see what kind of bogus result this logical error can cause, run this code:

Dim p As New Person3( Joe", Doe )

Dim e As New Employee3( Robert", Smith )

p.BirthDate = #12/31/1985#

e.BirthDate = #12/31/1985#

(Assuming that you run this code in the year 2003...)

The person can t vote yet (correct).

The employee is allowed to vote (incorrect).

Once you understand where the problem is, its solution is simple: you use the MyClass keyword to ensure that a method in a base class always uses the properties and meth ods in that base class (as opposed to their overridden version in the inherited class). Here s how to fix the problem in our example:

Part II:

Ensure that it always uses the nonoverridden version of the Age property. Return (MyClass.Age >= 18) End Get End Property

.NET lets you inherit from a class in a compiled DLL for which you neither have nor control the source code. This raises an interesting question: what happens if you extend the base class with a method or a property and then the author of the base class releases a new version that exposes a member with the same name Visual Basic copes with this situation in such a way that the application that uses the derived class isn t broken by changes in the base class. If the derived class has a mem ber with the same name as a member in the new version of the base class, you get a compilation warning, but you are still able to compile the two classes. In this case, the member in the derived class is said to be shadowing the member with the same name in the base class. Visual Basic offers three different syntax forms of shadowing:

A member in the derived class shadows all the members in the base class with the same name, regardless of their parameter signatures. As I ve explained, you get a compilation warning that doesn t prevent successful compilation (unless you select the Treat Compiler Warnings As Errors check box on the Build page of the project Property Pages dialog box). A member in the derived class marked with the Shadows keyword hides all the members in the base class with the same name, regardless of their signatures. The effect is exactly the same as in the preceding case. In addition, you don t get any compilation warning, so you should use the Shadows keyword to make it clear that you are intentionally shadowing one or more members in the base class. A member in the derived class marked with the Overloads keyword shadows only the member in the base class that has the same name and argument signature. (Note that you can t apply the Shadows and Overloads keywords to the same member.)

Shadowing can be confusing, so it s best to look at a concrete example:

Class AAA Sub DoSomething() Console.WriteLine( AAA.DoSomething ) End Sub Sub DoSomething(ByVal msg As String) Console.WriteLine( AAA.DoSomething({0})", msg) End Sub Sub DoSomething2() Console.WriteLine( AAA.DoSomething2 ) End Sub Sub DoSomething2(ByVal msg As String) Console.WriteLine( AAA.DoSomething2({0})", msg) End Sub

5:

End Class Class BBB Inherits AAA Overloads Sub DoSomething() Console.WriteLine( BBB.DoSomething ) End Sub Shadows Sub DoSomething2() Console.WriteLine( BBB.DoSomething2 ) End Sub End Class