| << Chapter < Page | Chapter >> Page > |
Listing 1 . Source code for class A.
using System;
class A {//this class is empty
}//end class A The class named A is empty. It was included in this example for the sole purpose of adding a layer of inheritance to the classhierarchy.
Listing 2 shows the definition of the class named B . This class extends the class named A as indicated by the colon that joins the B and the A on the first line.
Listing 2 . Source code for class B.
class B : A {
public void m() {Console.WriteLine("m in class B");
}//end method m()}//end class B The class named B defines a method named m . The behavior of the method is simply to display a message each time it iscalled.
Listing 3 shows the definition of the class named C , which also extends Object by default.
Listing 3 . Source code for class C.
class C {
//this class is empty}//end class C The class named C is also empty. It was included in this example as a sibling class for the class named A . Stated differently, it was included as a class that is not in the ancestral line ofthe class named B .
Listing 4 shows the beginning of the driver class named Polymorph02 , which also extends Object by default.
Listing 4 . Beginning of the class named Polymorph02.
public class Polymorph02 {
public static void Main() {Object var = new B(); This code instantiates an object of the class B and assigns the object's reference to a reference variable of type Object .
This assignment is allowable because Object is a superclass of B . In other words, the reference to the object of the class B is assignment-compatible with a reference variable of the type Object .
In this case, the reference of type B is automatically converted to type Object and assigned to the reference variable of type Object .
However, assignment compatibility is only part of the story. The simple fact that a reference is assignment-compatible with a reference variable of a giventype says nothing about what can be done with the reference after it is assigned to the reference variable.
For example, in this case, the reference variable that was automatically converted to type Object cannot be used directly to call the method named m on the object of type B. This is indicated in Listing 5 .
Listing 5 . Try to call method m on variable var.
//Following will not compile
//var.m(); An attempt to call the method named m on the referencevariable of type Object in Listing 5 resulted in the following compiler error:
error CS0117:
'object' does not contain a definition for 'm'
It was necessary to convert the statement to a comment in order to cause the program to compile successfully.
In order to use a reference of a class type to call a method, the method must be defined at or above that class in the class hierarchy. Stated differently,the method must either be defined in, or inherited into that class.
Notification Switch
Would you like to follow the 'Xna game studio' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|