C# Interface

Last Updated : 05 Apr, 2025

An interface is defined using the interface keyword, similar to a class. An Interface is a blueprint that outlines a group of methods, properties, events, or indexers that a class or struct must implement. Unlike classes, it contains only the declaration of the members. The implementation of the interface’s members will be given by a class that implements the interface implicitly or explicitly. There are some important points to remember, as mentioned below:

Interfaces cannot have private members.
By default, all the members of Interface are public and abstract.
The interface will always be defined with the help of the keyword interface
Interfaces cannot contain fields because they represent an implementation of data.
We can achieve multiple inheritance with the interface but not with the classes.
We can implement multiple interfaces in a single class separated by commas.

Example 1: Demonstrating the basic case for implementing the Interface.

// Demonstrate working of Interface  using System;   interface inter1 { 	// method having only declaration  	// with no definition 	void display(); }  // Implementing inteface in Geeks class Geeks : inter1 { 	// providing the body part of function  	public void display() 	{ 		Console.WriteLine("Demonstration of interface"); 	}    	// Main Method 	public static void Main(String[] args) 	{ 		Geeks t = new Geeks(); 		t.display(); 	} }

Output

Demonstration of interface

Example 2: Implementation of interfaces and the use of polymorphism in C#

// Using Interfaces and Polymorphism using System;  // interface declaration  interface Vehicle { 	// abstract method.  	void speedUp(int a); }  // class implements interface  class Bicycle : Vehicle { 	int speed;  	// Method increase speed  	public void speedUp(int increment) 	{ 		speed = speed + increment; 	}  	// Method check speed 	public void CheckSpeed() 	{ 		Console.WriteLine("speed: " + speed); 	} }  // class implements interface  class Bike : Vehicle { 	int speed;    	// to increase speed  	public void speedUp(int increment) 	{ 		speed = speed + increment; 	}  	public void CheckSpeed() 	{ 		Console.WriteLine("speed: " + speed); 	} }  class Geeks { 	public static void Main(String[] args) 	{ 		// creating an instance of Bicycle  		// doing some operations  		Bicycle bicycle = new Bicycle(); 		bicycle.speedUp(3);  		Console.WriteLine("Bicycle present state :"); 		bicycle.CheckSpeed();  		// creating instance of bike.  		Bike bike = new Bike(); 		bike.speedUp(4);  		Console.WriteLine("Bike present state :"); 		bike.CheckSpeed(); 	} }

Output

Bicycle present state : speed: 3 Bike present state : speed: 4

Advantages of Interfaces

Loose coupling: It is used to achieve loose coupling rather than concrete implementations, we can decouple classes and reduce interdependencies.
Abstraction: Interface helps to achieve full abstraction.
Maintainability: It helps to achieve component-based programming that can make code more maintainable.
Multiple Inheritance: Interface used to achieve multiple inheritance and abstraction.
Define architecture: Interfaces add a plug and play like architecture into applications.
Modularity: Promote a cleaner and more modular design. By separating the definition of behavior from the implementation

C# Interface

Mithun Kumar

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C# Interface

Advantages of Interfaces

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