6.5 Class inheritance

Inheritance in C# works in very much the same way as in Java, except for some differences in method overriding (see section 7.10). A class can inherit from only one superclass, and can implement multiple interfaces. Note the syntactical differences between the two languages though. ^[7]

^[7] C# resembles C++ syntax when it comes to inheritance. There is no extends or implements keyword like in Java, just the colon .

To inherit one class from another, use the following syntax:

 class <class_name>:<super_class> 

The code below contains two classes, Child and Parent . Child is a subclass of Parent , and inherits the DoSomething() method.

 1: using System;  2:  3: public class Parent{  4:   protected void DoSomething(){  5:     Console.WriteLine("inherited from Parent ");  6:   }  7: }  8: public class  Child:Parent  {  9:   static public void Main (){ 10:     new Child().DoSomething(); 11:   } 12: } 

Output:

 c:\expt>test inherited from Parent 

Like Java

• Every C# class must have one (and only one) superclass, except for System.Object which has no superclass. If no superclass is specified, a C# class implicitly subclasses System.Object directly.

• All class members are inherited except private members , constructors (both instance constructors and static constructors), and the special C# destructors. ^[8]

  ^[8] Destructors are special methods in C#. Constructors are invoked when an instance of a class is created “ destructors are invoked when that instance is destroyed and reclaimed.

• Multiple class inheritance ^[9] is not supported. You should look at implementing multiple interfaces as a rough substitute for multiple class inheritance.

  ^[9] Multiple inheritance, which is a notable feature in C++, brings several big problems, the biggest of which is resolution. When a class has two unrelated superclasses, and these two parents each contain an inheritable variable (or method) of the same name , a resolution mechanism is required to decide which should be inherited to the subclass. Single inheritance removes such problems. Instead of multiple inheritance, C# (and Java) supports multiple interface implementation. Like Java, a C# class can extend from one superclass, but implement as many interfaces as the programmer desires.

Unlike Java

• C# has a keyword virtual which is not found in Java. Methods, properties, and indexers can be declared virtual if they are intended to be overridden in subclasses. Only methods, properties, and indexers are class members whose implementation can be overridden in subclasses. (See section 7.7 for more information on virtual methods.)

• The direct superclass of a class type must be at least as accessible as the class type itself. For example, the following two line C# file does not compile:

   1: public class Child:Parent{} 2: class Parent{} 

  The compilation error given is:

  [View full width]

   
   [View full width] 
   test.cs(1,14): error CS0060: Inconsistent accessibility: base class 'Parent' is less  accessible than class 'Child' 

  Java does not have this requirement. The following Java file compiles properly:

   1: public class Child extends Parent{} 2: class Parent{} 

Additional note

The direct superclass of a user -defined class type must not be any of the following types:

• System.Array

• System.Delegate

• System.Enum

• System.ValueType

These are special classes of the BCL. ^[10]

^[10] All arrays in C# are automatically subclasses of System.Array . All delegates are subclasses of System.Delegate , all enums are subclasses of System.Enum and all structs are subclasses of System.ValueType . You cannot write your own class which extends these special System classes directly “ they are meant to act as common superclasses of these special types.