Introduction
Abstract classes are one of the essential behaviors provided by .NET. Commonly, you would like to make classes that only represent base classes, and don�t want anyone to create objects of these class types. You can make use of abstract classes to implement such functionality in C# using the modifier 'abstract'.
An abstract class means that, no object of this class can be instantiated, but can make derivations of this.
An example of an abstract class declaration is:
Collapseabstract class absClass
{
}
An abstract class can contain either abstract methods or non abstract methods. Abstract members do not have any implementation in the abstract class, but the same has to be provided in its derived class.
An example of an abstract method:
Collapse
abstract class absClass
{
public abstract void abstractMethod();
}
Also, note that an abstract class does not mean that it should contain abstract members. Even we can have anabstract class only with non abstract members. For example:
Collapse
abstract class absClass
{
public void NonAbstractMethod()
{
Console.WriteLine("NonAbstract Method");
}
}
A sample program that explains abstract classes:
Collapse
using System;
namespace abstractSample
{
abstract class absClass
{
public int AddTwoNumbers(int Num1, int Num2)
{
return Num1 + Num2;
}
public abstract int MultiplyTwoNumbers(int Num1, int Num2);
}
class absDerived:absClass
{
[STAThread]
static void Main(string[] args)
{
absDerived calculate = new absDerived();
int added = calculate.AddTwoNumbers(10,20);
int multiplied = calculate.MultiplyTwoNumbers(10,20);
Console.WriteLine("Added : {0},
Multiplied : {1}", added, multiplied);
}
public override int MultiplyTwoNumbers(int Num1, int Num2)
{
return Num1 * Num2;
}
}
}
In the above sample, you can see that the abstract class absClass contains two methods AddTwoNumbers andMultiplyTwoNumbers. AddTwoNumbers is a non-abstract method which contains implementation andMultiplyTwoNumbers is an abstract method that does not contain implementation.
The class absDerived is derived from absClass and the MultiplyTwoNumbers is implemented on absDerived. Within the Main, an instance (calculate) of the absDerived is created, and calls AddTwoNumbers andMultiplyTwoNumbers. You can derive an abstract class from another abstract class. In that case, in the childclass it is optional to make the implementation of the abstract methods of the parent class.
Example
Collapse
abstract class absClass1
{
public abstract int AddTwoNumbers(int Num1, int Num2);
public abstract int MultiplyTwoNumbers(int Num1, int Num2);
}
abstract class absClass2:absClass1
{
public override int AddTwoNumbers(int Num1, int Num2)
{
return Num1+Num2;
}
}
class absDerived:absClass2
{
public override int MultiplyTwoNumbers(int Num1, int Num2)
{
return Num1*Num2;
}
}
In the above example, absClass1 contains two abstract methods AddTwoNumbers and MultiplyTwoNumbers. The AddTwoNumbers is implemented in the derived class absClass2. The class absDerived is derived fromabsClass2 and the MultiplyTwoNumbers is implemented there.
Abstract properties
Following is an example of implementing abstract properties in a class.
Collapse
abstract class absClass
{
protected int myNumber;
public abstract int numbers
{
get;
set;
}
}
class absDerived:absClass
{
public override int numbers
{
get
{
return myNumber;
}
set
{
myNumber = value;
}
}
}
In the above example, there is a protected member declared in the abstract class. The get/set properties for the member variable myNumber is defined in the derived class absDerived.
Important rules applied to abstract classes
An abstract class cannot be a sealed class. I.e. the following declaration is incorrect.
Collapse
abstract sealed class absClass
{
}
Declaration of abstract methods are only allowed in abstract classes.
An abstract method cannot be private.
Collapse
private abstract int MultiplyTwoNumbers();
The access modifier of the abstract method should be same in both the abstract class and its derived class. If you declare an abstract method as protected, it should be protected in its derived class. Otherwise, the compiler will raise an error.
An abstract method cannot have the modifier virtual. Because an abstract method is implicitly virtual.
Collapse
public abstract virtual int MultiplyTwoNumbers();
An abstract member cannot be static.
Collapse
publpublic abstract static int MultiplyTwoNumbers();
Abstract class vs. Interface
An abstract class can have abstract members as well non abstract members. But in an interface all the members are implicitly abstract and all the members of the interface must override to its derived class.
An example of interface:
Collapse
interface iSampleInterface
{
int AddNumbers(int Num1, int Num2);
int MultiplyNumbers(int Num1, int Num2);
}
Defining an abstract class with abstract members has the same effect to defining an interface.
The members of the interface are public with no implementation. Abstract classes can have protected parts, static methods, etc.
A class can inherit one or more interfaces, but only one abstract class.
Abstract classes can add more functionality without destroying the child classes that were using the old version. In an interface, creation of additional functions will have an effect on its child classes, due to the necessary implementation of interface methods to classes.
The selection of interface or abstract class depends on the need and design of your project. You can make anabstract class, interface or combination of both depending on your needs.