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copied!<p>Many thanks to Jon and to mquander - you guys gave me enough information to help me solve this in a proper, generic way. Here's my solution, a single generic extension method that converts objects into hierarchy form:</p> <pre><code>public static IEnumerable<Node<T>> Hierarchize<T, TKey, TOrderKey>( this IEnumerable<T> elements, TKey topMostKey, Func<T, TKey> keySelector, Func<T, TKey> parentKeySelector, Func<T, TOrderKey> orderingKeySelector) { var families = elements.ToLookup(parentKeySelector); var childrenFetcher = default(Func<TKey, IEnumerable<Node<T>>>); childrenFetcher = parentId => families[parentId] .OrderBy(orderingKeySelector) .Select(x => new Node<T>(x, childrenFetcher(keySelector(x)))); return childrenFetcher(topMostKey); } </code></pre> <p>Utilizes this small node class:</p> <pre><code>public class Node<T> { public T Value { get; private set; } public IList<Node<T>> Children { get; private set; } public Node(T value, IEnumerable<Node<T>> children) { this.Value = value; this.Children = new List<Node<T>>(children); } } </code></pre> <p>It's generic enough to work for a variety of problems, including my text file issue. Nifty!</p> <p>****UPDATE****: Here's how you'd use it:</p> <pre><code>// Given some example data: var items = new[] { new Foo() { Id = 1, ParentId = -1, // Indicates no parent Position = 0 }, new Foo() { Id = 2, ParentId = 1, Position = 0 }, new Foo() { Id = 3, ParentId = 1, Position = 1 } }; // Turn it into a hierarchy! // We'll get back a list of Node<T> containing the root nodes. // Each node will have a list of child nodes. var hierarchy = items.Hierarchize( -1, // The "root level" key. We're using -1 to indicate root level. f => f.Id, // The ID property on your object f => f.ParentId, // The property on your object that points to its parent f => f.Position, // The property on your object that specifies the order within its parent ); </code></pre>

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