c#中查询表达式GroupBy的使用方法

public class Student
    {
        public int StuId { get; set; }

        public string ClassName { get; set; }

        public string StudentName { get; set; }
    }

List<Student> studentList = new List<Student>
            {
                new Student {ClassName = "软工一班", StudentName = "康巴一", StuId = 1},
                new Student {ClassName = "软工一班", StudentName = "康巴二", StuId = 2},
                new Student {ClassName = "软工一班", StudentName = "康巴三", StuId = 3},
                new Student {ClassName = "软工二班", StudentName = "康定一", StuId = 4},
                new Student {ClassName = "软工二班", StudentName = "康定二", StuId = 5},
                new Student {ClassName = "软工二班", StudentName = "康定三", StuId = 6},
            };

IEnumerable<IGrouping<string, Student>> studentGroup = studentList.GroupBy(s => s.ClassName);

foreach (IGrouping<string, Student> item in studentGroup)
            {
                
            }

foreach (IGrouping<string, Student> item in studentGroup)
            {
                Console.WriteLine(item.Key);
                foreach (var student in item)
                {
                    Console.WriteLine(student.StudentName);
                }
            }

public interface IGrouping<out TKey, out TElement> : IEnumerable<TElement>, IEnumerable
  {
    /// <summary>
    /// 获取 <see cref="T:System.Linq.IGrouping`2"/> 的键。
    /// </summary>
    /// 
    /// <returns>
    /// <see cref="T:System.Linq.IGrouping`2"/> 的键。
    /// </returns>
    [__DynamicallyInvokable]
    TKey Key { [__DynamicallyInvokable] get; }
  }