Question

I have two classes, named Post and Question. Question is defined as:

public class Question : Post
{
//...
}

My Question class does not override any members of Post, it just expresses a few other ones.

---

What I want to accomplish

I have an object of type Post, whose members are populated. Now, I want to convert it into a Question, so that I can add values for those few other members.

This is my current code, using an explicit cast conversion:

Post postToQuestion = new Post();

//Populate the Post...

Question ques = (Question)postToQuestion; //--> this is the error!

//Fill the other parts of the Question.

---

Problem

I am getting an InvalidCastException. What am I doing wrong?

Answer 1

The problem is that you can’t cast from parent to child. You can create a constructor for the child class that takes the parent class as a param:
Question ques = new Question(myPost);

You could also use the implicit operator to make it simple to do this:
Question ques = myPost;

http://www.codeproject.com/KB/cs/Csharp_implicit_operator.aspx

EDIT:
Actually, I just tried typing up a demo for you doing the implicit operator:

class Question : Post
{
    public Question()
    {
        //...
    }

    public Question(Post p)
    {
        // copy stuff to 'this'
    }

    public static implicit operator Question(Post p)
    {
        Question q = new Question(p);
        return q;
    }
}

but aparently C# doesn’t let you do implicit conversions with a base class.