Difference between Abstract Class and Interface in C#

An abstract class is a way to achieve abstraction in C#.

To declare an abstract class, we use the abstract keyword. 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. 

Example:

CSharp

// C# program to illustrate the
// concept of abstract class
using System;

// abstract class 'G'
public abstract class G {

    // abstract method 'gfg1()'
    public abstract void gfg1();
}

// class 'G' inherit
// in child class 'G1'
public class G1 : G {

    // abstract method 'gfg1()'
    // declare here with
    // 'override' keyword
    public override void gfg1()
    {
        Console.WriteLine("Class name is G1");
    }
}

// class 'G' inherit in
// another child class 'G2'
public class G2 : G {

    // same as the previous class
    public override void gfg1()
    {
        Console.WriteLine("Class name is  G2");
    }
}

// Driver Class
public class main_method {

    // Main Method
    public static void Main()
    {

        // 'obj' is object of class
        // 'G' class '
        // G' cannot
        // be instantiate
        G obj;

        // instantiate class 'G1'
        obj = new G1();

        // call 'gfg1()' of class 'G1'
        obj.gfg1();

        // instantiate class 'G2'
        obj = new G2();

        // call 'gfg1()' of class 'G2'
        obj.gfg1();
    }
}

Output :

Class name is G1
Class name is  G2

Like a class, Interface can have methods, properties, events, and indexers as its members. But interfaces will contain only the declaration of the members. The implementation of interface’s members will be given by the class who implements the interface implicitly or explicitly. Example:

CSharp

// C# program to illustrate the
// concept of interface
using System;

// A simple interface
interface interface1 {

    // method having only declaration
    // not definition
    void show();
}

// A class that implements the interface.
class MyClass : interface1 {

    // providing the body part of function
    public void show()
    {
        Console.WriteLine("Welcome to GeeksforGeeks!!!");
    }

    // Main Method
    public static void Main(String[] args)
    {

        // Creating object
        MyClass obj1 = new MyClass();

        // calling method
        obj1.show();
    }
}

Output:

Welcome to GeeksforGeeks!!!

Difference between Abstract Class and Interface

Abstract Class	Interface
It contains both declaration and implementation parts.	It contains only the declaration of methods, properties, events, or indexers. Since C# 8, default implementations can also be included in interfaces.
Multiple inheritance is not achieved by abstract class.	Multiple inheritance is achieved by interface.
It contain constructor.	It does not contain constructor.
It can contain static members.	It does not contain static members.
It can contain different types of access modifiers like public, private, protected etc.	It only contains public access modifier because everything in the interface is public.
The performance of an abstract class is fast.	The performance of interface is slow because it requires time to search actual method in the corresponding class.
It is used to implement the core identity of class.	It is used to implement peripheral abilities of class.
A class can only use one abstract class.	A class can use multiple interface.
If many implementations are of the same kind and use common behavior, then it is superior to use abstract class.	If many implementations only share methods, then it is superior to use Interface.
Abstract class can contain methods, fields, constants, etc.	Interface can only contains methods, properties, indexers, events.
The keyword “:” can be used for implementing the Abstract class.	The keyword “:” and “,” can be used for implementing the Interface.
It can be fully, partially or not implemented.	It should be fully implemented.
To declare abstract class , we use abstract keyword.	To declare interface, we use interface keyword.
Example of Abstract class:- public abstract class Fruits{ public abstract void Mango(); }	Example of Interface:- public interface Readable{ void read(); }