public static void c()
        {
            List <stu> lstu = new List <stu> {
                new stu {
                    id = 3, name = "zs", age = 1, pc = 0
                },
                new stu {
                    id = 3, name = "zs", age = 1, pc = 0
                },

                new stu {
                    id = 3, name = "zs", age = 2, pc = 0
                },
                new stu {
                    id = 2, name = "ls", age = 2, pc = 0
                },

                new stu {
                    id = 4, name = "ls", age = 3, pc = 2
                },
                new stu {
                    id = 4, name = "ls", age = 3, pc = 0
                },

                new stu {
                    id = 4, name = "lr", age = 4, pc = 2
                },
                new stu {
                    id = 4, name = "lr", age = 4, pc = 2
                },

                new stu {
                    id = 4, name = "lr", age = 5, pc = 2
                }
            };

            int c8 = lstu.GroupBy(p => p.age).Count() - lstu.Where(c => c.pc != 0).GroupBy(s => s.age).Count();
            IEnumerable <IGrouping <int, stu> > aaa = lstu.GroupBy(c => c.age);

            foreach (IGrouping <int, stu> sa in aaa)
            {
                //  List<stu> stua = sa.;
            }

            //lstu.Where(s => s.age == s.age && s.pc != 0).Count();
            //lstu.Select(s=>s.pc!=0)
            //   select * from b group by age
            List <stu> lstugroup     = lstu;
            List <stu> lstugroup0101 = lstu;

            string a  = "1,3,2,6,9,9";
            string b  = "1,3,5,7,9";
            string c1 = "1,3,5,7,9,7";
            string d  = "1,2,3,4,5,6";

            List <string> lsa   = a.Split(',').ToList();//数组转ToList集合(using System.Linq;)
            List <string> lsb   = b.Split(',').ToList();
            List <string> lsc   = c1.Split(',').ToList();
            List <string> lsd   = d.Split(',').ToList();
            List <string> lsstr = new List <string>()
            {
                "qq", "weibo", "zhihu"
            };
            List <int> lsint = new List <int>()
            {
                1, 2, 5
            };
            // 1 单个集合
            bool          bbh   = lsa.Contains("2");                            //是否包含
            bool          bbh2  = lstu.Exists(s => s.age == 1 && s.id == 5);
            List <string> lsqc  = lsa.Distinct().ToList();                      //一维去重
            List <string> lsqc4 = lstu.Select(p => p.name).Distinct().ToList(); //选择name属性并去重

            //List<string> lsqc5 = lstu.Count(s=>s.age)

            //2,排序,求和
            lsa.Sort();                                                 //排序,无返回值;
            lstu = lstu.OrderBy(s => s.id).ThenBy(s => s.age).ToList(); // 以id,age排序
            decimal sum  = lsint.Sum();                                 //合计,list 必须为int等类型
            decimal sum2 = lstu.Sum(s => s.age);

            // 2 set操作 两个集合:交差并 集合;
            bool          bjj  = lsa.Intersect(lsb).Count() > 0; //判断是否有交集
            List <string> lsjj = lsa.Intersect(lsb).ToList();    //求交集   1,3,9

            List <string> lscj  = lsa.Except(lsb).ToList();      //求相对差集  相对las得2,6
            List <string> lsbj2 = lsa.Union(lsc).ToList();       //求并集(并保留重复项) 1,2,3,6,9,9,1,3,5,7,9
            List <string> lsbj1 = lsa.Union(lsb).ToList();       //求并集(并剔除重复项)  1,2,3,5,6,7,9

            lstu = lstu.Skip(2).Take(3).ToList();                //从第二项开始取3个项目;System.Linq.Enumerable
            stu t = lstu.Find(s => s.id > 2);

            lstu = lstu.Where(s => s.id > 2).ToList();

            //3 group by
            var ab2 = (from p in lstugroup group p by p.name into g select new { name = g.Key }).ToList();                //单属性distinct
            var ab3 = (from p in lstugroup group p by new { p.name, p.age } into g select new { name = g.Key }).ToList(); //多属性distinct

            //4 过滤
            List <stu> ab4 = lstugroup.GroupBy(p => p.id).Select(p => new stu {
                id = p.Key, name = p.FirstOrDefault().name
            }).ToList();                                                                                                              //单属性distinct并默认其他属性

            // 5 linq where findAll 的比较_形式
            List <stu> ab5 = (from i in lstugroup where i.id > 3 select i).ToList();
            List <stu> ab6 = lstugroup.Where(s => s.id > 3).ToList();
            List <stu> ab7 = lstugroup.FindAll(s => s.id > 3);//直接返回该对象,不需要toList()

            // 6 linq where findAll 的比较_性能  》数据比较功能 where≈findAll > linq
            List <stu> stuceshi = new List <stu>();

            for (int i = 0; i < 100000; i++)
            {
                stuceshi.Add(new stu {
                    id = i, name = "name" + i, age = i, mm = "zs"
                });
            }
            //10w次比对数据
            //List<stu> ab8 = (from i in stuceshi where i.mm =="zs" select i).ToList();//11.1742ms
            //List<stu> ab9 = lstugroup.Where(s => s.mm == "zs").ToList();//0.3283ms
            //List<stu> ab10 = lstugroup.FindAll(s => s.mm == "zs");//0.3225ms
            // 8千DM_DORM对象筛选 三者性能略同
            string name1;

            System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
            sw.Start();
            for (int i = 0; i < 1000000; i++)
            {
                //name1 = stuceshi.Find(s => s.id == 99999) == null ? "" : stuceshi.Find(s => s.id == 99999).name;
                //stuceshi = stuceshi.Where(s => s.name == "").ToList();
                stuceshi = stuceshi.Where(s => s.name.Length == 0).ToList();
            }
            sw.Stop();
            string ts2 = sw.Elapsed.TotalMilliseconds.ToString();

            // 7 处理find异常.大量数据时,自己处理异常效率是try catch的6倍;
            string name3 = lstu.Find(s => s.id == 8) == null ? "" : lstu.Find(s => s.id == 8).name;

            //string name1 = stuceshi.Find(s => s.id == -1) == null ? "" : stuceshi.Find(s => s.id == -1).name;  //3.9594
            //string name1 = stuceshi.Find(s => s.id == 99999) == null ? "" : stuceshi.Find(s => s.id == 99999).name;  //7.0533
            #region try catch情况 18403
            //for (int i = 0; i > -1000; i--)  // 18403.0000
            //{
            //    try //22.5663
            //    {
            //        name1 = stuceshi.Find(s => s.id == -1).name;
            //    }
            //    catch (Exception)
            //    {

            //        name1 = "";
            //    }
            //}
            #endregion
            #region 自处理情况 3250
            //for (int i = 0; i > -1000; i--) 时间0.3274~8490.0000(全比较是8490)
            //{
            //    name1 = stuceshi.Find(s => s.id == 99999) == null ? "" : stuceshi.Find(s => s.id == 99999).name;
            //}
            #endregion

            // 8双问号操作符
            // string strParam= Request.Params["param"]?? "";
            int?cc    = null;
            int name4 = cc ?? 0;

            // 9 StartsWith()
            lstugroup = lstugroup.Where(s => s.name.StartsWith("l")).ToList(); // 筛选name以l开头;
            //lstugroup =lstugroup.Where(s => s.name.StartsWith("l")).

            //10 遍历集合
            int ii = 0;
            foreach (stu item in lstugroup)
            {
                ii = lstugroup.IndexOf(item);
            }

            //11 处理Where,找不到是count=0而不报错。
            lstugroup = lstugroup.Where(s => s.name.StartsWith("m")).ToList(); // 筛选name以l开头;

            //12

            var g2 = lstugroup0101.GroupBy(x => new { x.id, x.age });

            //lstugroup0101= g2.Where(v => v.Key ==3).ToList();
            List <stu> dddd = new List <stu>();
            foreach (var item in g2)
            {
                foreach (var item2 in item)
                {
                    dddd.Add(item2);
                }
            }
        }//断点处