标准用法：
public class Student { public string Name { get; set; } public bool Graduation { get; set; } public int Age { get; set; } public int Score { get; set; } public string City { get; set; } public override string ToString() { return $"姓名：{Name},年龄：{Age},分数：{Score},是否毕业：{Graduation},城市：{City}"; } } class Program { static void Main(string[] args) { var students = new List<Student> { new Student{Name = "张三",Age = 15,Score = 94,Graduation = true,City = "北京"}, new Student{Name = "李四",Age = 17,Score = 47,Graduation = false,City = "北京"}, new Student{Name = "王二",Age = 19,Score = 77,Graduation = false,City = "广州"}, new Student{Name = "孙五",Age = 14,Score = 14,Graduation = false,City = "上海"} }; var groupByResult = students.GroupBy(x => x.City,student=>new{student.Name,student.City}); foreach (var item in groupByResult) { Console.WriteLine($"分组城市：{item.Key}"); foreach (var student in item) { Console.WriteLine($"姓名：{student.Name},城市：{student.City}"); } } } }输出结果：
标准用法：
分组城市：北京 姓名：张三,城市：北京 姓名：李四,城市：北京 分组城市：广州 姓名：王二,城市：广州 分组城市：上海 姓名：孙五,城市：上海方法 3
public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector);本方法重载与之前的方法不一样，没有了 elementSelector 参数，变成了 resultSelector 参数，并且其返回值也由 IEnumerable<IGrouping<TKey, TSource>> 变成了 IEnumerable<YResult> 。
在这个方法当中，我们只需要通过 resultSelector 委托即可指定需要输出的数据类型，这里该委托的 TKey 参数是分组的列/属性，而 IEnumerable<TSource> 则是每个分组结果的关联序列。
标准用法：
public class Student { public string Name { get; set; } public bool Graduation { get; set; } public int Age { get; set; } public int Score { get; set; } public string City { get; set; } public override string ToString() { return $"姓名：{Name},年龄：{Age},分数：{Score},是否毕业：{Graduation},城市：{City}"; } } class Program { static void Main(string[] args) { var students = new List<Student> { new Student{Name = "张三",Age = 15,Score = 94,Graduation = true,City = "北京"}, new Student{Name = "李四",Age = 17,Score = 47,Graduation = false,City = "北京"}, new Student{Name = "王二",Age = 19,Score = 77,Graduation = false,City = "广州"}, new Student{Name = "孙五",Age = 14,Score = 14,Graduation = false,City = "上海"} }; var groupByResult = students.GroupBy(x => x.City, (key, enumerable) => { return new { City = key, Avg = enumerable.Average(x => x.Score), Max = enumerable.Max(x => x.Score) }; }); foreach (var student in groupByResult) { Console.WriteLine($"城市：{student.City},平均分：{student.Avg},最高分：{student.Max}"); } } }输出结果：
城市：北京,平均分：70.5,最高分：94 城市：广州,平均分：77,最高分：77 城市：上海,平均分：14,最高分：14方法 4
public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector);最后一个方法与之前的方法一样，不过多了 elementSelector，与第二个方法一样，用于指定要选择的属性字段，排除其他字段。
标准用法：
public class Student { public string Name { get; set; } public bool Graduation { get; set; } public int Age { get; set; } public int Score { get; set; } public string City { get; set; } public override string ToString() { return $"姓名：{Name},年龄：{Age},分数：{Score},是否毕业：{Graduation},城市：{City}"; } } class Program { static void Main(string[] args) { var students = new List<Student> { new Student{Name = "张三",Age = 15,Score = 94,Graduation = true,City = "北京"}, new Student{Name = "李四",Age = 17,Score = 47,Graduation = false,City = "北京"}, new Student{Name = "王二",Age = 19,Score = 77,Graduation = false,City = "广州"}, new Student{Name = "孙五",Age = 14,Score = 14,Graduation = false,City = "上海"} }; var groupByResult = students.GroupBy(x => x.City, x=>new{x.Name,x.City,x.Age,x.Score},(key, enumerable) => { return new { City = key, Avg = enumerable.Average(x => x.Score), Max = enumerable.Max(x => x.Score), AvgAge = enumerable.Average(x=>x.Age) }; }); foreach (var student in groupByResult) { Console.WriteLine($"城市：{student.City},平均分：{student.Avg},最高分：{student.Max},平均年龄：{student.AvgAge}"); } } }