Question

I’ve done this before – just can’t remember the trick.

If i have an abstract class:

public abstract class Post

And a set of deriving classes:

public class Photo : Post

I want to force the deriving classes to implement a method called Validate(), but at the same time providing core validation at the Post level.

I can create a method: public abstract void Validate() in Post, which would force the deriving classes to implement the method, but then how do i perform the Post (base) validation?

The end result is i want to be able to do this:

public class BLL
{
   public void AddPost(Post post)
   {
       post.Validate(); // includes "Post" validation, any deriving validation.
       repository.Add(post);
   }
}

How can i do it?

Answer 1

Here is what you want:

public abstract class Post {

    // Default validation technique
    public void Validate()
    {
        // your base validation
        // Then call the specific validation technique
        ExtraValidate();
    }

    // Forces subclasses to provide validation technique
    protected abstract void ExtraValidate();
}

This will force base classes to implement a validation technique, and the base Validate will get called by external users.

It is impossible to make a method abstract, and provide a default implementation.