When to use abstract classes and when to use interfaces in C#

Question

Possible Duplicates:
Abstract classes vs Interfaces
Interface vs Base class

Hi,

In C# when should we use interfaces and when should we use abstract classes? Please explain with example

Answer 1

use interfaces when you don't when your describing a contract and abstract classes when your implmenting functionality that will be shared among deriving classes.

To examples of usage could be template pattern and Strategy pattern

Answer 2

Abstract class, if you want to implement basic functionality and some properties/methods that must be overwritten by the implementer:

public abstract class Entity
{
     public int Id { get; set;}

     public bool IsNew { get { return Id == 0; } }

     public abstract DoSomething(int id); // must be implemented by concrete class
}

Interface, if you want to define, which properties/methods a class must contain, without implementing any functionality:

public interface IRepository
{
    object Get(int id);
    IEnumerable<object> GetAll();
}