C#实现无限层级树

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            var list = new List<Menu>()
            {
                new Menu () { Id = 1, Name = "主菜单1", PId = 0  },
                new Menu () { Id = 2, Name = "下级菜单11", PId = 1  },
                new Menu () { Id = 3, Name = "主菜单2", PId = 0  },
                new Menu () { Id = 4, Name = "下级菜单21", PId = 3 },
                new Menu () { Id = 5, Name = "下下级菜单212", PId = 4  }
            };

            var dictMenus = new Dictionary<int, Menu>(list.Count);

            foreach (var menu in list)
            {
                dictMenus.Add(menu.Id, menu);
            }

            foreach (var value in dictMenus.Values)
            {
                if (dictMenus.ContainsKey(value.PId))
                {
                    if (dictMenus[value.PId].Children == null)
                        dictMenus[value.PId].Children = new List<Menu>();
                    dictMenus[value.PId].Children.Add(value);
                }
            }
            var result = dictMenus.Values.Where(t => t.PId == 0).ToList();
        }
    }

    public class Menu
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int PId { get; set; }
        public List<Menu> Children { get; set; }
    }
}