freeCodeCamp/guide/english/csharp/interface/index.md

title
Interface

An interface is similar to a class or struct but without implementation for its members. An interface declares a contract or a behavior that implementing classes require. It may declare only properties, methods and events with NO access modifiers. All the declared members must be implemented in the inherited class, otherwise you will have a compile-time error. The convention is to start the name of the interface with the letter I, such as IMyInterface or IUserOptions. You define an interface by using the interface keyword. All members of an interface are: implicitly abstract, implicitly public, cannot declare an access modifier such as protected, internal private etc... An Interface can: * Inherit from other interfaces. * Inherit from multiple interfaces at the same time * Contain only methods, properties, events, and indexers. An Interface can not: * Inherit from a class. * Have implementation. * Have access modifiers other than public. * Be instantiated.

Using interfaces allows you to change your implementation in our project without breaking other parts. You only have to change the one place where the object is created. Interfaces also ensure that any new members added to the interface are implimented in all derived classes by throwing errors at compile time.

As interfaces do not implement any logic by themselves, they are a great tool for programmers to program against a type rather than a concrete object. This is very useful in keeping code loosely coupled. This behavior of interfaces made them imperative for implementation of Design Patterns.

Interface Example:

public interface IUserFavoriteFood
{
  void AddFood();
  Task<User> EatFavoriteFood(int id);
}

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Interface inheritance and implementation:

public class UserHungry : IUserFavoriteFood
{
  public AddFood()
  {
    // Implementation:
    // A method to add food.
  }

  public Task<User> EatFavoriteFood(int id)
  {
    // Implementation:
    // A method to Eat food by id.
  }
}

Every implementation can be different:

public class AnotherUserHungry : IUserFavoriteFood
{
  public AddFood()
  {
    // DIFFERENT Implementation:
    // A method to add vegan food.
  }
  
  public Task<User> EatFavoriteFood(int id)
  {
    // DIFFERENT Implementation:
    // A method to Eat only vegan food by id.
  }
}