Object-Oriented Programming in VS .NET

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Object-Oriented Programming
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exposes a ReadOnly property, you can t make it writable by overriding it in the derived class. Similarly, you can t define a read-write property that overrides a WriteOnly prop erty in the base class. Along the same lines, if you re overriding a default member in the base class, the method in the derived class must be the default member in the derived class and requires the Default keyword. Notice that you can t override fields, constants, or shared members defined in the base class.
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Override Variations
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By default, a method marked with the Overrides keyword is itself overridable, so you never need both the Overrides and Overridable keywords in the same procedure defini tion, even though using both is legal. You need the NotOverridable keyword to explicitly tell the compiler that an overridden method isn t overridable in derived classes:
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This procedure overrides a procedure in the base class, but this
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procedure can t be overridden in any class that inherits from the current
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class.
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NotOverridable Overrides Sub MyProc()
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End Sub
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When adding a member with the same name but a different signature, you need neither the Overrides keyword in the derived class nor the Overridable keyword in the base class. For example, if the Employee class contains a CompleteName method with one argument, it doesn t override the parameterless method with the same name in the Person class. Therefore, no Overridable or Overrides keyword is necessary. Oddly enough, however, the method in the derived class does require the Overloads keyword:
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...(In the Person (base) class)... Note: no Overridable keyword Function CompleteName() As String Return FirstName & & LastName End Function ...(In the Employee (derived) class)...
Note: no Overrides keyword, but Overloads is required.
Overloads Function CompleteName(ByVal title As String) As String
Return title & & LastName & , & FirstName End Function
The general rule is therefore as follows: you don t need the Overloads keyword when a class defines multiple members with identical names, but you need the Overloads keyword in the derived class when the derived class exposes a new member with the same name but a different signature. If the derived class additionally overrides a base class member with the same signature, you re forced to use both the Overloads and the Overrides keywords for this member. You also must honor the rule (stated in
5:
Inheritance
4) that says if you use Overloads for a member in a class, you must use it for all the members with the same name. The compiler can generate more efficient code when calling nonoverridable methods instead of overridable (virtual) methods, so you might want to avoid using the Overrid able keyword if you can. For example, the JIT compiler can inline regular methods but not virtual methods. (Inlining is an optimization technique through which the compiler moves code from the called method into the caller s procedure.) In addition, allocating an object that contains virtual methods takes slightly longer than the allocation of an object that has no virtual methods. An informal benchmark shows that an overridable method can be twice as slow as a nonoverridable one, even though the difference in absolute terms is small and you need a loop with millions of iterations to make it evident. While we re talking about performance, remember that calling a virtual method on a value type forces the compiler to consider it a reference type, which causes the object to be boxed in the heap and therefore degrades the overall execution speed. For exam ple, this happens when you call the ToString method on a value type such as a Struc ture, as you can see by looking at the IL code produced by such a call.
The MyBase Keyword
The MyBase keyword is useful when you want to reference a field, property, or method of the base object. If a member hasn t been overridden in the derived class, the expressions Me.membername and MyBase.membername refer to the same member and execute the same code. However, when membername has been redefined in the inherited class, you need the MyBase keyword to access the member as defined in the base class. Consider the following method:
...(In the Person (base) class)... Overridable Function CompleteName() As String Return FirstName & & LastName End Function
Now, let s assume that the Employee class overrides this method to prefix the complete name with the employee s title. Here s a not-so-smart implementation of this method:
...(In the Employee (derived) class)... Public Title As String Overrides Function CompleteName() As String If Title <> Then CompleteName = Title & CompleteName &= FirstName & & LastName End Function
The preceding solution isn t optimal because it doesn t reuse any code in the base class. In this particular case, the code in the base class is just a string concatenation operation, but in a real class it might be dozens or hundreds of statements. Worse, if you later change or improve the implementation of the CompleteName function in the
Part II:
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