Interface

refer to this link

http://www.c-sharpcorner.com/UploadFile/3d39b4/interface-in-C-Sharp-part-1/

In a plain term, Interface is a contact having only the method or function definitions. Sometimes interfaces are considered as application protocols.

Interfaces must be used in an application where there can be multiple implementations and you want to provide a good level of abstraction. If you provide an interface as a functional protocol, others have to provide the actual implementations to respect your functional protocol.

Think about a situation, you want to validate an object based upon certain rules, but the validations may be changed in different situations. In this case you have to provide an interface. I provide below an example.

//Situation 1
Validator validator = new Situation1ValidatorImpl1();

validator.validate(yourObject);

//Situation 2
Validator validator = new Situation2ValidatorImpl2();

validator.validate(yourObject);

//Situation 3
Validator validator = new Situation3ValidatorImpl3();

validator.validate(yourObject);

In the above case Validator is an Interface where as Situation1ValidatorImpl1,Situation2ValidatorImpl2,Situation3ValidatorImpl3 are different implementations in different context. You can also mark that the code "validator.validate(yourObject);" remains static through out your application.

The primary objective of usage of Interface is to provide a level of abstraction in your application.