Question

I have a database table where all the records are linked among each other i.e. something similar to the image bellow:

alt text http://img691.imageshack.us/img691/6772/38103866.png

As you can see on the diagram, a record can be root and can have 1 or more children, where each child stores in its ParentID property the ID property of its parent. I was wondering if someone can help me with constructing a LINQ expression that returns all the nodes starting with the last child and finishing with the root. What I mean is the following. Starting from Node 4 (ID = 4) I have to move up to Node 2 (ID = 2), then Node 1 and then Node 0, thus skipping Node 3. I hope I am clear enough, but if something needs clarifying let me know.

Answer 1

This’ll get the job done:

static IEnumerable<Node> ListParents(IEnumerable<Node> list, int? ID)
{
    var current = list.Where(n => n.ID == ID).FirstOrDefault();
    if (current == null)
        return Enumerable.Empty<Node>();
    return Enumerable.Concat(new []{current}, ListParents(list, current.ParentID));
}

This is assuming a Node class like:

class Node
{
    public int ID;
    public int? ParentID;
}

Note that if your ParentID relationships cause a cycle, this function will recurse infinitely.