Visual Basic.NET How to Program (PDF)

438 Object-Oriented Programming: Polymorphism Chapter 10
Methods cmdSortAscending_Click (lines 82–93) and cmdSort-
Descending_Click (lines 96–107) are invoked when the user clicks the Sort
Ascending and Sort Descending buttons, respectively. In method
cmdSortAscending_Click, line 87 passes to CDelegateBubbleSort method
SortArray the unsorted mElementArray and a reference to method SortAscending.
Keyword AddressOf returns a reference to method SortAscending.
Visual Basic implicitly converts the method reference into a delegate object that contains
the method reference. The second argument of line 87 is equivalent to
New CDelegateBubbleSort.Comparator(AddressOf SortAscending)
which explicitly creates a CDelegateBubbleSort Comparator delegate object. In
method cmdSortDescending_Click, line 101 passes the unsorted mElementArray
and a reference to method SortDescending to method SortArray. We continue
to use delegates in Chapters 12–14, when we discuss event handling and multithreading.
In Chapter 11, Exception Handling, we discuss how to handle problems that might
occur during a program’s execution. The features presented in Chapter 11 enable programmers
to write more robust and fault-tolerant programs.
SUMMARY
Polymorphism enables us to write programs in a general fashion to handle a wide variety of existing
and future related classes.
One means of processing objects of many different types is to use a Select Case statement to
perform an appropriate action on each object based on that object’s type.
Polymorphic programming can eliminate the need for Select Case logic.
When we override a base class’s method in a derived class, we hide the base class’s implementation
of that method.
With polymorphism, new types of objects not even envisioned when a system is created may be
added without modification to the system (other than the new class itself).
Polymorphism allows one method call to cause different actions to occur, depending on the type
of the object receiving the call. The same message assumes “many forms”—hence, the term polymorphism.
With polymorphism, the programmer can deal in generalities and let the executing program concern
itself with the specifics.
When we apply the MustInherit keyword to a class, we cannot create instances of that class.
Instead, we create classes that inherit from the MustInherit class and create instances of those
derived classes.
Any class with a MustOverride method in it must, itself, be declared MustInherit.
Although we cannot instantiate objects of MustInherit base classes, we can declare references
to MustInherit base classes. Such references can manipulate instances of the derived classes
polymorphically.
A method that is declared NotOverridable cannot be overridden in a derived class.
Methods that are declared Shared and methods that are declared Private are implicitly NotOverridable.
A class that is declared NotInheritable cannot be a base class (i.e., a class cannot inherit from
a NotInheritable class).