Nice & universal way to convert List of items to Tree

If you want to have universal method you”ll need an additional class:

public class TreeItem<T>
{
    public T Item { get; set; }
    public IEnumerable<TreeItem<T>> Children { get; set; }
}

Then use it with this helper:

internal static class GenericHelpers
{
    /// <summary>
    /// Generates tree of items from item list
    /// </summary>
    /// 
    /// <typeparam name="T">Type of item in collection</typeparam>
    /// <typeparam name="K">Type of parent_id</typeparam>
    /// 
    /// <param name="collection">Collection of items</param>
    /// <param name="id_selector">Function extracting item's id</param>
    /// <param name="parent_id_selector">Function extracting item's parent_id</param>
    /// <param name="root_id">Root element id</param>
    /// 
    /// <returns>Tree of items</returns>
    public static IEnumerable<TreeItem<T>> GenerateTree<T, K>(
        this IEnumerable<T> collection,
        Func<T, K> id_selector,
        Func<T, K> parent_id_selector,
        K root_id = default(K))
    {
        foreach (var c in collection.Where(c => EqualityComparer<K>.Default.Equals(parent_id_selector(c), root_id)))
        {
            yield return new TreeItem<T>
            {
                Item = c,
                Children = collection.GenerateTree(id_selector, parent_id_selector, id_selector(c))
            };
        }
    }
}

Usage:

var root = categories.GenerateTree(c => c.Id, c => c.ParentId);

Testing:

static void Test(IEnumerable<TreeItem<category>> categories, int deep = 0)
{
    foreach (var c in categories)
    {
        Console.WriteLine(new String('\t', deep) + c.Item.Name);
        Test(c.Children, deep + 1);
    }
}
// ...
Test(root);

Output

Sport
    Balls
    Shoes
Electronics
    Cameras
        Lenses  
        Tripod
    Computers
        Laptops
Empty