The C# Language in C#.NET

Encoder Quick Response Code in C#.NET The C# Language

class TwoDShape { double pri_width; // now private double pri_height; // now private // Properties for width and height public double Width { get { return pri_width; } set { pri_width = value < 0 -value : value; } } public double Height { get { return pri_height; } set { pri_height = value < 0 -value : value; } } public void ShowDim() { ConsoleWriteLine("Width and height are " + Width + " and " + Height); } } // A derived class of TwoDShape for triangles class Triangle : TwoDShape { public string Style; // style of triangle // Return area of triangle public double Area() { return Width * Height / 2; } // Display a triangle's style public void ShowStyle() { ConsoleWriteLine("Triangle is " + Style); } } class Shapes2 { static void Main() { Triangle t1 = new Triangle(); Triangle t2 = new Triangle(); t1Width = 40; t1Height = 40; t1Style = "isosceles"; t2Width = 80; t2Height = 120; t2Style = "right"; ConsoleWriteLine("Info for t1: "); t1ShowStyle(); t1ShowDim(); ConsoleWriteLine("Area is " + t1Area());

11:

ConsoleWriteLine(); ConsoleWriteLine("Info for t2: "); t2ShowStyle(); t2ShowDim(); ConsoleWriteLine("Area is " + t2Area()); } }

In this version, the properties Width and Height provide access to the private members, pri_width and pri_height, which actually store the values Therefore, even though pri_width and pri_height are private to TwoDShape, their values can still be set and obtained through their corresponding public properties When referring to base and derived classes, sometimes the terms superclass and subclass are used These terms come from Java programming What Java calls a superclass, C# calls a base class What Java calls a subclass, C# calls a derived class You will commonly hear both sets of terms applied to a class of either language, but this book will continue to use the standard C# terms C++ also uses the base-class/derived-class terminology

As just explained, a private member of a base class is not accessible to a derived class This would seem to imply that if you wanted a derived class to have access to some member in the base class, it would need to be public Of course, making the member public also makes it available to all other code, which may not be desirable Fortunately, this implication is untrue because C# allows you to create a protected member A protected member is public within a class hierarchy, but private outside that hierarchy A protected member is created by using the protected access modifier When a member of a class is declared as protected, that member is, with one important exception, private The exception occurs when a protected member is inherited In this case, a protected member of the base class becomes a protected member of the derived class and is, therefore, accessible to the derived class Therefore, by using protected, you can create class members that are private to their class but that can still be inherited and accessed by a derived class Here is a simple example that uses protected:

// Demonstrate protected using System; class B { protected int i, j; // private to B, but accessible by D public void Set(int a, int b) { i = a; j = b; } public void Show() { ConsoleWriteLine(i + " " + j); } }

Part I:

class D : B { int k; // private // D can access B's i and j public void Setk() { k = i * j; } public void Showk() { ConsoleWriteLine(k); } } class ProtectedDemo { static void Main() { D ob = new D(); obSet(2, 3); // OK, known to D obShow(); // OK, known to D obSetk(); // OK, part of D obShowk(); // OK, part of D } }

In this example, because B is inherited by D and because i and j are declared as protected in B, the Setk( ) method can access them If i and j had been declared as private by B, then D would not have access to them, and the program would not compile Like public and private, protected status stays with a member no matter how many layers of inheritance are involved Therefore, when a derived class is used as a base class for another derived class, any protected member of the initial base class that is inherited by the first derived class is also inherited as protected by a second derived class Although protected access is quite useful, it doesn t apply in all situations For example, in the case of TwoDShape shown in the preceding section, we specifically want the Width and Height values to be publicly accessible It s just that we want to manage the values they are assigned Therefore, declaring them protected is not an option In this case, the use of properties supplies the proper solution by controlling, rather than preventing, access Remember, use protected when you want to create a member that is accessible throughout a class hierarchy, but otherwise private To manage access to a value, use a property