Implementing a tree in c# managing parent-child

Question

I am implementing a tree think of it as a folder structure so I have a class that looks like:

public class Folder
{
    //Various Props like Name etc.
    public IList<Folder> Children{get;}
    public Folder Parent {get;}
}

Now what I want is to be able to walk up and down the tree so given a root I can find a leaf, and given a leaf I can find the root node. So each child needs a parent. Now the question is what is the best way to add a new node to the tree. I have used two solutions in the past:

1. Add an AddChild(Folder) method to Folder which handles adding the folder, and can set the parent. The problem with this is I now have to lock my Children collection so you can't bypass this method.
2. Create my own Children collection which will be given a reference back to the instance, so it can handle setting the parent on the add. The problem with this I have to implement a new collection.
3. Use a collection which has events when items are added or deleted.

I am curious what patterns people generally use, and then if anyone has any suggestions for my specific use case. I am using nHibernate to persist my tree to SQL server. I'd rather not implement a custom collection as it's a lot of code to get this to work for something which is a very small part of my application.

Answer 1

After looking on MSDN you could try this:

List<Folder> children;

public ReadOnlyCollection<Folder> Children
{
    get { return this.children.AsReadOnly(); }
}

If your private member must be declared as an IList then we can copy that into a list and then return it. But I really don't see a problem with using a concrete implementation as a private member. Changing the implementation later won't break compatibility.

IList<Folder> children;

public ReadOnlyCollection<Folder> Children
{
    get 
    { 
        return new List<Folder>(this.children).AsReadOnly(); 
    }
}