Question

I was given this question at a job interview recently and couldn’t figure out how to do it elegantly. Ever since, it has been nagging away at me and I can’t work out if its a lack of knowledge about some ‘modern’ technique/technology I’m unaware of or if I’m just stupid. Any advice would be very welcome.

The Problem

Imagine a simple class hierarchy:

abstract class Person {
    public string Name { get; set; }
}

class Child : Person { }

class Parent : Person {
    public List<Person> Children { get; set; }
}

class Ancestor : Parent { }

The problem is how to traverse a hierarchy of such objects and to print out all the people encountered. So for the following scenario:

Ancestor myAncestor = new Ancestor {    
    Name = "GrandDad",
    Children = new List<Person> { 
        new Child { Name = "Aunt" },
        new Child { Name = "Uncle" },
        new Parent {
            Name = "Dad", 
            Children = new List<Person> { 
                new Child { Name = "Me" }, 
                new Child { Name = "Sister" } 
            }
        }
    }
};

the output should be something like:

GrandDad  
-    Aunt  
-    Uncle  
-    *Dad  
         -Me  
         -Sister

All the processing needs to be done within a single method that accepts a single parameter of type Ancestor.

I implemented, almost without thinking, a simple recursive solution but of course because of the way the objects involved relate to each other things aren’t as simple as all that.

Try as I might I cannot think of a clean way of doing this and my post-interview Googlings have suggested I need to be doing something that is (to me, with only a working knowledge of LINQ and List<T>) something considerably more technically advanced than the sort of web-dev coding I’ve been doing for the last decade or so. Is this the case? Or should I be thinking of getting out of software development on the grounds that I’m rubbish at it?

Update

Thanks to you all for your responses/suggestions. I’ve accepted @Daniel Hilgarth’s answer primarily because it was the only one I could genuinely understand 😮

Answer 1

I agree with Marc’s comment saying that this type system is non-sense. Still, you can solve it with delegates. That’s a bit of cheating, because basically they are nothing more than methods, but here we go:

void PrintFamily(Ancestor a)
{
    Action<Parent, int> printParent = null;
    printParent = (parent, level) => 
    {
        var indentation = new string(' ', level * 4);
        var indentationChildren = new string(' ', (level + 1) * 4);
        Console.WriteLine(indentation + parent.Name);
        foreach(var child in parent.Children)
        {
            if(child is Child)
                Console.WriteLine(indentationChildren + child.Name);
            else if(child is Parent)
            {
                printParent((Parent)child, level + 1);
            }
        }
    };

    printParent(a, 0);
}