Flat Hierarchy Conversion

FlatToHierarchy.cs

        private static List<Category> FlatToHierarchy(List<Category> list, int parentID = 0)
        {
            return (from l in list
                    where l.ctgParentID == parentID
                    select new Category
                    {
                        ctgID = l.ctgID,
                        ctgParentID = l.ctgParentID,
                        ctgName = l.ctgName,
                        childCategories = FlatToHierarchy(list, l.ctgID),
                        ctgExist = l.ctgExist
                    }).ToList();
        }
        private static IEnumerable<Category> HierarchyToFlat(Category parent)
        {
            yield return parent;
            foreach (var child in parent.childCategories)
                foreach (var relative in HierarchyToFlat(child))
                    yield return relative;
        }