How an interface can replace a class?

Question

I created a Method called GetStudentMarks(). The return-type of this method is generic List<StudentMark>.The code works well even when i replaced the List<StudentMark> with generic IList<StudentMark>. How can an interface replace a class while interface contain only the declarations?

Answer 1

An interface cannot replace a class. It's just a blueprint for a class that has some implementation that corresponds by the guidelines that are set by the interface. So, you mostly will have one interface and than 1 or multiple classes that have some implementation for that interface like so:

public interface IMyInterface{
 IList<string> SomeList { get; }
}

public class MyClass : IMyInterface {
  public IList<string> SomeList {
    get { 
      return new List<string>(){ "a", "b" , "c" }; 
    }
  }
}