lambda GroupBy

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);
        foreach (var item in groupByResult)
        {
            Console.WriteLine($"分组城市：{item.Key}");
            foreach (var student in item)
            {
                Console.WriteLine(student.ToString());
            }
        }
    }
}




分组城市：北京
姓名：张三,年龄：15,分数：94,是否毕业：True,城市：北京
姓名：李四,年龄：17,分数：47,是否毕业：False,城市：北京
分组城市：广州
姓名：王二,年龄：19,分数：77,是否毕业：False,城市：广州
分组城市：上海
姓名：孙五,年龄：14,分数：14,是否毕业：False,城市：上海

 对多个元素分组

 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 = "李1",Age = 17,Score = 47,Graduation = false,City = "北京"},
            new Student{Name = "李四",Age = 15,Score = 47,Graduation = false,City = "北京"},
            new Student{Name = "王二",Age = 19,Score = 77,Graduation = false,City = "广州"},
            new Student{Name = "王1",Age = 22,Score = 77,Graduation = false,City = "广州"},
            new Student{Name = "王3",Age = 22,Score = 44,Graduation = false,City = "广州"},
            new Student{Name = "王4",Age = 19,Score = 33,Graduation = false,City = "广州"},
            new Student{Name = "孙1",Age = 20,Score = 14,Graduation = false,City = "上海"},
            new Student{Name = "孙2",Age = 14,Score = 14,Graduation = false,City = "上海"},
            new Student{Name = "孙3",Age = 20,Score = 14,Graduation = false,City = "上海"},
            new Student{Name = "孙4",Age = 14,Score = 14,Graduation = false,City = "上海"}
        };

            var groupByResult = students.GroupBy(s=>new {s.City,s.Age });//返回六个集合，每个集合有item

            foreach (var item in groupByResult)
            {
                foreach(var s in item)
                {
                    Console.WriteLine($"城市：{s.City},分数：{s.Score},name：{s.Name}");
                }
            }
城市：北京,分数：94,name：张三
城市：北京,分数：47,name：李四
城市：北京,分数：47,name：李四
城市：北京,分数：47,name：李1
城市：广州,分数：77,name：王二
城市：广州,分数：33,name：王4
城市：广州,分数：77,name：王1
城市：广州,分数：44,name：王3
城市：上海,分数：14,name：孙1
城市：上海,分数：14,name：孙3
城市：上海,分数：14,name：孙2
城市：上海,分数：14,name：孙4

简单的对分组操作，返回重组后的对象集合

其第一个参数的意思与之前一样，用于指定分组条件。第二个 elementSelector 参数则是与 Select() 方法类似，可以指定输出的分组结果类型。
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}");
            }
        }
    }
}

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

对分组后的集合 再进行操作

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}");
        }
    }
}


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

 与上一个类似，不过 限定了后面选择的属性，感觉实际没啥用

 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}");
            }