FindInTree - recursively searches a tree of objects. Expects as parameters a lambda to retrieve an item's identifier and a lambda to get a node's children.

FindInTree.cs

  /// <summary>
        /// Recursively searches a tree structure for a specific node.
        /// </summary>
        /// <typeparam name="TChild">The node's type.</typeparam>
        /// <typeparam name="TIdentifier">The node identifier's type</typeparam>
        /// <param name="root">The root node to search under.</param>
        /// <param name="identifier">The identifier of the node to find.</param>
        /// <param name="childSelector">A function that returns a node's children in the tree.</param>
        /// <param name="identifierSelector">A function that returns a node's identifier</param>
        /// <returns>Either a <cref>TChildType</cref> instance whose identifier matches the one specified, or null if not found.</returns>
        public static TChild FindInTree<TChild, TIdentifier>
            (this TChild root, TIdentifier identifier, Func<TChild, IEnumerable<TChild>> childSelector, Func<TChild, TIdentifier> identifierSelector)
            where TChild : class
        {
            if (root == null)
                return null;

            if (Equals(identifierSelector(root), identifier))
            {
                return root;
            }

            foreach (var child in childSelector(root))
            {
                var foundChild = child.FindInTree(identifier, childSelector, identifierSelector);
                if (foundChild != null)
                    return foundChild;
            }

            return null;
        }