LINQ分组时如何进行组内排序？另外如何仅返回每组中满足条件的行（如Id最大的）？

1、定义演示数据类型：

    public class TestData
    {
        public int GrpId { get; private set; }
        public int Id { get; set; }
        public int IdMod5 { get; private set; }
        public string Name { get; set; }

        public TestData(int id, string name)
        {
            Random _random = new Random();
            this.Id = id;
            this.Name = name;
            this.GrpId = this.Id % 3;
            this.IdMod5 = this.Id % 6;
        }
    }

假定已初始化了如图的数据：

2、演示分组时组内排序和组外排序方法：

            Console.WriteLine("\r\n演示组内按Name降序和组外按Key.RID升序:");
            var numberGroups = from n in testdatas
                               orderby n.Name descending
                               group n by new { n.IdMod5, RID = n.GrpId } into g
                               orderby g.Key.RID
                               select new { Remainder = g.Key, Numbers = g };
            foreach (var g in numberGroups)
            {
                Console.WriteLine("Numbers with a remainder of {0} when divided by 5:", g.Remainder);
                foreach (var n in g.Numbers)
                {
                    Console.WriteLine("Id:{0}   Name:{1}    RID:{2}  IdMod5:{3}", n.Id, n.Name, n.GrpId, n.IdMod5);
                }
            }

运行结果见下图：

3、演示先分组，然后每组里面仅取满足条件的行组：

            Console.WriteLine("\r\n演示先分组，然后每组里面仅取Id最大的行组:");
            var lets = from a in testdatas
                       group a by a.GrpId into grp
                       let maxId = grp.Max(a => a.Id)
                       from row in grp
                       where row.Id == maxId
                       select row;

            foreach (var g in lets)
            {
                Console.WriteLine("Id:{0}   Name:{1}    RID:{2}  IdMod5:{3}", g.Id, g.Name, g.GrpId, g.IdMod5);
            }

运行结果如下图：

 演示代码