LINQ：对结果进行分组（MSDN）

帮助程序类和数据源示例

public class StudentClass
    {
        #region data
        public enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear };
        public class Student
        {
            public string FirstName { get; set; }
            public string LastName { get; set; }
            public int ID { get; set; }
            public GradeLevel Year;
            public List<int> ExamScores;
        }

        protected static List<Student> students = new List<Student>
    {
        new Student {FirstName = "Terry", LastName = "Adams", ID = 120,
            Year = GradeLevel.SecondYear,
            ExamScores = new List<int>{ 99, 82, 81, 79}},
        new Student {FirstName = "Fadi", LastName = "Fakhouri", ID = 116,
            Year = GradeLevel.ThirdYear,
            ExamScores = new List<int>{ 99, 86, 90, 94}},
        new Student {FirstName = "Hanying", LastName = "Feng", ID = 117,
            Year = GradeLevel.FirstYear,
            ExamScores = new List<int>{ 93, 92, 80, 87}},
        new Student {FirstName = "Cesar", LastName = "Garcia", ID = 114,
            Year = GradeLevel.FourthYear,
            ExamScores = new List<int>{ 97, 89, 85, 82}},
        new Student {FirstName = "Debra", LastName = "Garcia", ID = 115,
            Year = GradeLevel.ThirdYear,
            ExamScores = new List<int>{ 35, 72, 91, 70}},
        new Student {FirstName = "Hugo", LastName = "Garcia", ID = 118,
            Year = GradeLevel.SecondYear,
            ExamScores = new List<int>{ 92, 90, 83, 78}},
        new Student {FirstName = "Sven", LastName = "Mortensen", ID = 113,
            Year = GradeLevel.FirstYear,
            ExamScores = new List<int>{ 88, 94, 65, 91}},
        new Student {FirstName = "Claire", LastName = "O'Donnell", ID = 112,
            Year = GradeLevel.FourthYear,
            ExamScores = new List<int>{ 75, 84, 91, 39}},
        new Student {FirstName = "Svetlana", LastName = "Omelchenko", ID = 111,
            Year = GradeLevel.SecondYear,
            ExamScores = new List<int>{ 97, 92, 81, 60}},
        new Student {FirstName = "Lance", LastName = "Tucker", ID = 119,
            Year = GradeLevel.ThirdYear,
            ExamScores = new List<int>{ 68, 79, 88, 92}},
        new Student {FirstName = "Michael", LastName = "Tucker", ID = 122,
            Year = GradeLevel.FirstYear,
            ExamScores = new List<int>{ 94, 92, 91, 91}},
        new Student {FirstName = "Eugene", LastName = "Zabokritski", ID = 121,
            Year = GradeLevel.FourthYear,
            ExamScores = new List<int>{ 96, 85, 91, 60}}
    };
        #endregion

        //Helper method, used in GroupByRange.
        public static int GetPercentile(Student s)
        {
            double avg = s.ExamScores.Average();
            return avg > 0 ? (int)avg / 10 : 0;
        }


        public static List<Student> GetStudents()
        {
            return students;

        }

        public void QueryHighScores(int exam, int score)
        {
            var highScores = from student in students
                             where student.ExamScores[exam] > score
                             select new { Name = student.FirstName, Score = student.ExamScores[exam] };

            foreach (var item in highScores)
            {
                Console.WriteLine($"{item.Name,-15}{item.Score}");
            }
        }
    }

按单个属性分组示例

            var students = StudentClass.GetStudents();

            #region 按单个属性分组示例

            Console.WriteLine("Group by a single property in an object:");

            var queryFirstLetters =
                  from student in students
                  group student by student.LastName[0];

            foreach (var studentGroup in queryFirstLetters)
            {
                Console.WriteLine($"Key: {studentGroup.Key}");
                // Nested foreach is required to access group items.
                foreach (var student in studentGroup)
                {
                    Console.WriteLine($"	{student.LastName}, {student.FirstName}");
                }
            }
            #endregion

queryFirstLetters 关联：

参考：

对查询结果进行分组 https://docs.microsoft.com/zh-cn/dotnet/csharp/linq/group-query-results