我应该为抽象类创建接口吗？

Question

Is there any practial reason to create interface for abstract class? I encountered such thing:

public interface IEntity<T> 
{
    T Id { get; set; }
}

public abstract class BaseEntity { 
}

public abstract class Entity<T> : BaseEntity, IEntity<T> 
{
    public virtual T Id { get; set; }
}

and i really don't understand what is the difference between that and this code, because IEntity is not a thing i whould use more than once:

public abstract class BaseEntity { 
}

public abstract class Entity<T> : BaseEntity
{
    public virtual T Id { get; set; }
}

Thanks!

Answer 1

Since the BaseEntity class does actually add any properties of methods, yes, it is completely different from the interface. The interface defines a property Id of type T , and that interface (contract) can be used in other locations in your application.

The base class as it is now, is useless IMHO.

A base class which would implement the interface for you is something that would be usable, like this:

public interface IEntity<T> {
    T Id { get; set; }
}

public abstract class BaseEntity<T>: IEntity<T> { 
    public virtual T Id { get; set; }
}

public abstract class Entity<T> : BaseEntity<T> {
    // No need to implement the Id property, we already have it inherited
}

Answer 2

Here is the same thing with correct answer that i wanted to know: c# Abstract Class implementing an Interface

Thanks everyone for help.