非常好用的一个分组扩展方法

 public static class GroupHelper
    {
        public static List<List<object>> Group(this List<object> source, Func<List<object>, bool> limitFunc)
        {
            return Group<object>(source, limitFunc);
        }

        public class t {
            public string key { get; set; }
        }
        public static object test()
        {
            var list = new List<t> { 
                new t { key = "b" }, new t { key = "c" },new t { key = "b" },
                new t { key = "a" },new t { key = "c" },new t { key = "e6" }, new t { key = "e9" }, new t { key = "c" },
                new t { key = "e1" }, new t { key = "e4" },new t { key = "e7" },new t { key = "e10" },
                 new t { key = "b" },new t { key = "e5" },new t { key = "e8" },new t { key = "e11" },
                new t { key = "e3" },  new t { key = "e13" },new t { key = "a" },new t { key = "e2" },new t { key = "e12" },
            };
            var res = list.GroupBy(m => m.key).OrderByDescending(m => m.Count()).SelectMany(m => m).ToList();
            return res.Select(m=>m.key) ;
            //"b", "b", "b", "c", "c", "c", "a", "a", "e6", "e9", "e1", "e4", "e7", "e10", "e5", "e8", "e11", "e3", "e13", "e2", "e12"
            var source = "a a b b b c c c d d e1 e2 e3 e4 e5 e6 e7 e8 e9 e10 e11 e12 e13".Split(' ').ToList();
            var groups = source.Group(r => (!(r.Count == 2 && r.Distinct().Count() == 2) && r.Count > 1 && r.Distinct().Count() == 2) || (r.Count == 13 && r.Distinct().Count() == 13));
            return groups;
            //result:
            //aa
            //bbb
            //ccc
            //dd
            //e1-e12
            //e13
        }
        /// <summary>
        /// 分组
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="source">源</param>
        /// <param name="limitFunc">项限制</param>
        /// <returns></returns>
        public static List<List<T>> Group<T>(this List<T> source, Func<List<T>, bool> limitFunc)
        {
            var result = new List<List<T>>();

            List<T> resItem = null;
            var inter = source.GetEnumerator();
            foreach (var item in source)
            {
                if (resItem == null) resItem = new List<T>();

                resItem.Add(item);
                //超限移除当前项并移交 result，创建新项。
                if (limitFunc(resItem.ToList()))
                {
                    resItem.Remove(item);
                    result.Add(resItem);
                    resItem = new List<T> { item };
                }
            }
            if (resItem != null && resItem.Count > 0)
            {
                result.Add(resItem);
            }

            return result;
        }
}