Rendering a hierarchy using LINQ?

Question

Let say we have a class

Category
{
   ID,
   Name,
   ParentID
}

and a List

1, 'Item 1', 0
2, 'Item 2', 0
3, 'Item 3', 0
4, 'Item 1.1', 1
5, 'Item 3.1', 3
6, 'Item 1.1.1', 4
7, 'Item 2.1', 2

Can we using LINQ to render a tree like:

Item 1
 Item 1.1
  Item 1.1.1
Item 2
 Item 2.1
Item 3
 Item 3.1

Any help is appreciated!

Answer 1

Here's the "LINQ-only" version:

Func<int, int, string[]> build = null;
build = (p, n) =>
{
    return (from x in categories
            where x.ParentID == p
            from y in new[]
            {
                "".PadLeft(n)+ x.Name
            }.Union(build(x.ID, n + 1))
            select y).ToArray();
};
var lines = build(0, 0);

Yes, it's recursive LINQ.

---

Per NVA's request, here's the way to make all "orphan" records become root records:

Func<IEnumerable<int>, int, string[]> build = null;
build = (ps, n) =>
{
    return (from x in categories
            where ps.Contains(x.ParentID)
            from y in new[]
    {
        "".PadLeft(n)+ x.Name
    }.Union(build(new [] { x.ID }, n + 1))
            select y).ToArray();
};

var roots = (from c in categories
             join p in categories on c.ParentID equals p.ID into gps
             where !gps.Any()
             orderby c.ParentID
             select c.ParentID).Distinct();

var lines = build(roots, 0);