Ineterface with example in C#

Interface Definition: An interface is basically a contract or service which will implemented by other classes.
This implements the rules which will forced to use by derived classes.
Interface has only declarations of methods and properties. They do not have body in method they
have only the signatures.
Declaring Interfaces:

public interface IUser
{
void AddUser(User user);
void EditUser(int Id)
User GetUserByID(int Id);
}
Implementation:

public class User:IUser
{
public User GetUserByID(int Id)
{
 var context=...................;
var user=context.Users.find(Id);
return user;
}
public EditUser(int Id)
{
var context=...................;
var user=context.Users.find(Id);
context.Entry(user).State=EntityState.Modified;
context.SaveChanges();
}
public void AddUser(User user)
{
var context=...................;
context.Users.Add(user);
context.SaveChanges();
}
}