C# | Abstract Classes - Tutorialspoint.dev

Abstraction in C# is the process to hide the internal details and showing only the functionality. The abstract modifier indicates the incomplete implementation. The keyword abstract is used before the class or method to declare the class or method as abstract. Also the abstract modifier can be used with indexers, events and properties.

Example:

public abstract void geek();
// this indicates the method 'geek()' is abstract
 
abstract class gfg
// this indicates the class 'gfg' is abstract

Abstract Method: A method which is declared abstract, has no “body” and declared inside the abstract class only.

Syntax:

public abstract void geek();
// the method 'geek()' is abstract

Abstract Class: This is the way to achieve the abstraction in C#. An Abstract class is never intended to be instantiated directly. This class must contain at least one abstract method, which is marked by the keyword or modifier abstract in the class definition. The Abstract classes are typically used to define a base class in the class hierarchy.

Syntax:

abstract class gfg{}
// class 'gfg' is abstract

Important Points:

Generally, we use abstract class at the time of inheritance.
A user must use the override keyword before the method which is declared as abstract in child class, the abstract class is used to inherit in the child class.
An abstract class cannot be inherited by structures.
It can contains constructors or destructors.
It can implement functions with non-Abstract methods.
It cannot support multiple inheritance.
It can’t be static.

Example 1: Program to show the working of an abstract class

// C# program to show the  
// working of abstract class 
using System; 
  
// abstract class 'GeeksForGeeks' 
public abstract class GeeksForGeeks { 
  
    // abstract method 'gfg()' 
    public abstract void gfg(); 
      
} 
  
// class 'GeeksForGeeks' inherit 
// in child class 'Geek1' 
public class Geek1 : GeeksForGeeks 
{ 
  
    // abstract method 'gfg()'  
    // declare here with  
    // 'override' keyword 
    public override void gfg() 
    { 
        Console.WriteLine("class Geek1"); 
    } 
} 
  
// class 'GeeksForGeeks' inherit in  
// another child class 'Geek2' 
public class Geek2 : GeeksForGeeks 
{ 
  
    // same as the previous class 
    public override void gfg() 
    { 
        Console.WriteLine("class Geek2"); 
    } 
} 
  
// Driver Class 
public class main_method { 
  
    // Main Method 
    public static void Main() 
    { 
  
        // 'g' is object of class 
        // 'GeeksForGeeks' class ' 
        // GeeksForGeeks' cannot 
        // be instantiate 
        GeeksForGeeks g; 
  
        // instantiate class 'Geek1' 
        g = new Geek1(); 
          
        // call 'gfg()' of class 'Geek1' 
        g.gfg(); 
        
        // instantiate class 'Geek2'   
        g = new Geek2(); 
        
        // call 'gfg()' of class 'Geek2' 
        g.gfg(); 
          
    } 
} 

Output:

class Geek1
class Geek2

Example 2: Program to calculate the area of a square using abstract class and abstract method

// C# program to calculate the area  
// of a Square using abstract class 
// and abstract method 
using System; 
  
// declare class 'AreaClass' 
// as abstract 
abstract class AreaClass 
{ 
    // declare method  
    // 'Area' as abstract 
    abstract public int Area(); 
} 
  
// class 'AreaClass' inherit 
// in child class 'Square' 
class Square : AreaClass 
{ 
    int side = 0; 
  
    // constructor 
    public Square(int n) 
    { 
        side = n; 
    } 
  
    // the abstract method  
    // 'Area' is overridden here 
    public override int Area() 
    { 
        return side * side; 
    } 
} 
  
class gfg { 
  
    // Main Method 
    public static void Main() 
    { 
        Square s = new Square(6); 
        Console.WriteLine("Area  = " + s.Area()); 
    } 
} 

Output:

Area  = 36

Following are some important observations about abstract classes in C#

1) An Abstract class does not mean that it only contain abstract methods. An Abstract class can also contain non-abstract methods also.

Syntax:

abstract class gfg
{
    public void geek()
    {
        Console.WriteLine("'geek()' is non-abstract method");
    }
}

Example:

// C# program to show the working of  
// the non-abstract method in the  
// abstract class 
using System; 
  
abstract class AbstractClass { 
  
    // Non abstract method 
    public int AddTwoNumbers(int Num1, int Num2) 
    { 
        return Num1 + Num2; 
    } 
  
    // An abstract method which  
    // overridden in the derived class 
    public abstract int MultiplyTwoNumbers(int Num1, int Num2); 
      
} 
  
// Child Class of AbstractClass 
class Derived : AbstractClass { 
  
    // implementing the abstract  
    // method 'MultiplyTwoNumbers' 
    // using override keyword, 
    public override int MultiplyTwoNumbers(int Num1, int Num2) 
    { 
        return Num1 * Num2; 
    } 
} 
  
// Driver Class 
class geek { 
  
    // Main Method 
    public static void Main() 
    { 
   
        // Instance of the derived class 
        Derived d = new Derived(); 
  
        Console.WriteLine("Addition : {0}
Multiplication :{1}", 
                                          d.AddTwoNumbers(4, 6), 
                                    d.MultiplyTwoNumbers(6, 4)); 
    } 
} 

Output:

Addition : 10
Multiplication :24

2) Abstract class can also work with get and set accessors.

Example:

// C# program to show the working  
// of abstract class with the  
// get and set accessors 
using System; 
  
abstract class absClass { 
  
    protected int myNumber; 
  
    public abstract int numbers 
    { 
        get; 
        set; 
    } 
} 
  
class absDerived : absClass { 
  
    // Implementing abstract properties 
    public override int numbers 
    { 
        get
        { 
            return myNumber; 
        } 
        set
        { 
            myNumber = value; 
        } 
    } 
} 
  
// Driver Class 
class geek { 
  
    // Main Method 
    public static void Main() 
    { 
        absDerived d = new absDerived(); 
        d.numbers = 5; 
        Console.WriteLine(d.numbers); 
    } 
} 

Output:

C# | Abstract Classes

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