Ошибка при создании бинарного дерева - C#

Формулировка задачи:

Ошибка: Error 1 'treeParentChild.Program.CategoryCollection' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'. 'treeParentChild.Program.CategoryCollection.GetEnumerator()' cannot implement 'System.Collections.IEnumerable.GetEnumerator()' because it does not have the matching return type of 'System.Collections.IEnumerator'. C:\Users\...\Desktop\treeParentChild\treeParentChild\Program.cs 56 15 treeParentChild Код программы:

#region Directives
using System;
using System.Collections.Generic;
#endregion
#region mainCode : treeParentChild
namespace treeParentChild
{
    class Program
    {
        public static void Main(string[] args)
        {
            /* Tree Structure
 
               a
                d
                 e   
               b
                f
                i
                 j
               c
                g
                 h
            */
 
            var a = new Category("a", null);
            var b = new Category("b", null);
            var c = new Category("c", null);
            var d = new Category("d", "a");
            var e = new Category("e", "d");
            var f = new Category("f", "b");
            var g = new Category("g", "c");
            var h = new Category("h", "g");
            var i = new Category("i", "b");
            var j = new Category("j", "i");
            var k = new Category("k", "z");
 
            var list = new CategoryCollection { k, j, i, h, g, f, e, d, c, b, a };
            foreach (var category in list.SortForTree())
            {
                Console.WriteLine("Name: {0}; Parent: {1}", category.Name, category.ParentCategoryID);
            }
 
        }
        class Category
        {
            public string ParentCategoryID { get; set; }
            public string Name { get; set; }
            public Category(string name, string parentCategoryID)
            {
                Name = name;
                ParentCategoryID = parentCategoryID;
            }
        }
 
        class CategoryCollection : IEnumerable<Category>
        {
            private List<Category> list = new List<Category>();
 
            public void Add(Category category)
            {
                list.Add(category);
            }
 
            public IEnumerable<Category> SortForTree()
            {
                var target = new Dictionary<string, Category>();
 
                SortForTree(list, target);
 
                return target.Values;
            }
 
            private void SortForTree(List<Category> source, Dictionary<string, Category> target)
            {
                var temp = new List<Category>();
 
                foreach (var c in source)
                {
                    if (c.ParentCategoryID == null || (target.ContainsKey(c.ParentCategoryID) && !target.ContainsKey(c.Name)))
                    {
                        target.Add(c.Name, c);
                    }
                    else
                    {
                        if (source.Exists(o => o.Name == c.ParentCategoryID))
                        {
                            temp.Add(c);
                        }
                    }
                }
 
                if (temp.Count > 0) SortForTree(temp, target);
            }
 
            #region IEnumerable<Category> Members
 
            public IEnumerator<Category> GetEnumerator()
            {
                return list.GetEnumerator();
            }
 
            #endregion
        }
 
    }
}
#endregion

Решение задачи: «Ошибка при создании бинарного дерева»

textual
                            Листинг программы 
                        

                            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
            {
                throw new NotImplementedException();
            }
                        