interfaces vs abstract classes c# code example

Example 1: c# abstract class

abstract class Shape
{
    public abstract int GetArea();
}

class Square : Shape
{
    int side;

    public Square(int n) => side = n;

    // GetArea method is required to avoid a compile-time error.
    public override int GetArea() => side * side;

    static void Main() 
    {
        var sq = new Square(12);
        Console.WriteLine($"Area of the square = {sq.GetArea()}");
    }
}
// Output: Area of the square = 144

Example 2: c# abstract class and interfaces

// ------------------- ABSTRACT vs INTERFACES ------------------------ //
// Both are used to define the architecture of the application


// ---------- ABSTRACT CLASS --------- //
// -> The methods can have an implementation (be defined)
// -> It's elements are private by default
// -> It can contain fields
// -> It can inherit from another abstract class or interface 
// -> It can have abstract and non-abstract methods. The abstract classes have to be implemented in the child class. 

public abstract class AbsParent { 
  
  	private int count = 0;
  
    public void foo1()    
    {
    	Console.WriteLine("Hi there, I'm a normal method that will be inherited!");
    }
  
    public abstract void foo2();  // You can't have an implementation for this abstract method 
  
    public virtual void foo3()    
    {
    	Console.WriteLine("Hi there, I'm a virtual method!");
    }  
} 


// --------- INTERFACES --------- //
// -> It's not a class, it's an entity
// -> The methods can't have an implementation
// -> It's elements are public by default and can't have acesso modifiers 
// -> It can't contain fields like for example, "private int count", but it can contain properties 
// -> It can only inherit from another interface 
// -> A class can inherit from multiple interfaces
// -> A class inheriting from an interface, has to implement all it's methods 

public interface IParent { 
  
  string myProterty{get; set;} 
  
  void foo1(); // You can't have an implementation for these methods
  void foo2(); 
  
} 



// -------- INHERITANCES --------- // 

// class 'AbsParent' inherit in child class 'Child1' 
public class Child1 : AbsParent { 
  
    public override void foo2()    // The implementation of the AbsParent abstract methods is mandatory 
    { 
        Console.WriteLine("Only in the child/derived class, can I be defined!"); 
    } 
    
  
  	public override void foo3() 
    { 
        Console.WriteLine("Class name is Child1"); 
    } 
} 
  

// class 'IParent' inherit in another child class 'Child2' 
public class Child2 : IParent { 
 
    public void foo1()    // The implementation of all the IParent methods is mandatory 
    { 
        Console.WriteLine("Only in the child/derived class, can I be defined!"); 
    } 
    
  
  	public void foo2() 
    { 
        Console.WriteLine("Only in the child/derived class, can I be defined!"); 
    } 

} 



// ---------- MAIN --------- //
public class main_method { 
  
    // Main Method 
    public static void Main() 
    { 
        AbsParent obj = new AbsParent();  // ERROR!! You can't create an object of an abstract class
                                          // it is used only for inheritance purposes. The same logic 
                                          // goes to interfaces.
    } 
}

Example 3: abstract classes vs interfaces

Interface 
1) Interface contains only abstract methods 
2) Access Specifiers for methods in interface
must be public
3) Variables defined must be public , static ,
final
4) Multiple Inheritance in java is implemented
using interface
5) To implement an interface we use
implements keyword

Abstract Class
1) Abstract class can contain abstract methods,
concrete methods or both
2) Except private we can have any access
specifier for methods in abstract class.
3) Except private variables can have any access
specifiers
4)We cannot achieve multiple inheritance using
abstract class.
5)To implement an interface we use extends
keyword