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C# 30: A Beginner s Guide
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public void ShowDim() { ConsoleWriteLine("Width and height are " + Width + " and " + Height); } public virtual double Area() { ConsoleWriteLine("Area() must be overridden"); return 00; } } // A derived class of TwoDShape for triangles class Triangle : TwoDShape { string Style; // A default constructor public Triangle() { Style = "null"; } // Constructor that takes style, width, and height public Triangle(string s, double w, double h) : base(w, h, "triangle") { Style = s; } // Construct an isosceles triangle public Triangle(double x) : base(x, "triangle") { Style = "isosceles"; } // Construct a copy of a Triangle object public Triangle(Triangle ob) : base(ob) { Style = obStyle; } // Override Area() for Triangle public override double Area() { return Width * Height / 2; }
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Override Area( ) for Triangle Area( ) is now virtual
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public void ShowStyle() { ConsoleWriteLine("Triangle is " + Style); } } // A derived class of TwoDShape for rectangles class Rectangle : TwoDShape {
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8: Inheritance
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// Constructor that takes width and height public Rectangle(double w, double h) : base(w, h, "rectangle"){ } // Construct a square public Rectangle(double x) : base(x, "rectangle") { } // Construct an object from an object public Rectangle(Rectangle ob) : base(ob) { } // Return true if rectangle is square public bool IsSquare() { if(Width == Height) return true; return false; } // Override Area() for Rectangle public override double Area() { return Width * Height; } } class DynShapes { static void Main() { TwoDShape[] shapes = new TwoDShape[5]; shapes[0] shapes[1] shapes[2] shapes[3] shapes[4] = = = = = new new new new new Triangle("right", 80, 120); Rectangle(10); Rectangle(10, 4); Triangle(70); TwoDShape(10, 20, "generic");
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Override Area( ) for Rectangle
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for(int i=0; i < shapesLength; i++) { ConsoleWriteLine("object is " + shapes[i]Name); ConsoleWriteLine("Area is " + shapes[i]Area()); ConsoleWriteLine(); } } }
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The proper version of Area( ) is called for each shape
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The output from the program is shown here:
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object is triangle Area is 48 object is rectangle
C# 30: A Beginner s Guide
Area is 100 object is rectangle Area is 40 object is triangle Area is 245 object is generic Area() must be overridden Area is 0
Let s examine this program closely First, as explained, Area( ) is declared as virtual in the TwoDShape class and is overridden by Triangle and Rectangle Inside TwoDShape, Area( ) is given a placeholder implementation that simply informs the user that this method must be overridden by a derived class Each override of Area( ) supplies an implementation that is suitable for the type of object encapsulated by the derived class Thus, if you were to implement an ellipse class, for example, then Area( ) would need to compute the area of an ellipse There is one other important feature in the preceding program Notice in Main( ) that shapes is declared as an array of TwoDShape objects However, the elements of this array are assigned Triangle, Rectangle, and TwoDShape references This is valid because a base class reference can refer to a derived class object The program then cycles through the array, displaying information about each object Although quite simple, this program illustrates the power of both inheritance and method overriding The type of object stored in a base class reference variable is determined at runtime and acted on accordingly If an object is derived from TwoDShape, then its area can be obtained by calling Area( ) The interface to this operation is the same, no matter what type of shape is being used
Using Abstract Classes
Sometimes you will want to create a base class that defines only a generalized form that will be shared by all of its derived classes, leaving it to each derived class to fill in the details Such a class determines the nature of the methods that the derived classes must implement, but does not, itself, provide an implementation of one or more of these methods One way this situation can occur is when a base class is unable to create a meaningful implementation for a method This is the case with the version of TwoDShape used in the preceding example The definition of Area( ) is simply a placeholder It will not compute and display the area of any type of object As you will see as you create your own class libraries, it is not uncommon for a method to have no meaningful definition in the context of its base class You can handle this situation two ways One way, as shown in the previous example, is to simply have it report a warning message While this approach can be useful in certain situations such as debugging it is not usually appropriate You may have methods that must be overridden by the derived class in order for the derived class to have any meaning Consider the class Triangle It has no meaning if Area( ) is not defined In this case, you want some way to ensure that a derived class does, indeed, override all necessary methods C# s solution to this problem is the abstract method
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