c# Linq及Lambda表达式应用经验之 GroupBy 分组 count ,排序

引用：http://www.cnblogs.com/han1982/p/4138163.html

示例1：

GroupBy 分组在List<>泛型中的应用

原表：

按姓名Nam 分组后结果：

对DATATABLE 进行LAMDA查询时必须在项目的引用中添加 System.Data.DataSetExtensions

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }  

        List<Person> persons1 = new List<Person>();  

        private void Form1_Load(object sender, EventArgs e)
        {
            initForm();
        }
        private void initForm()
        {//窗体初始化  

            persons1.Add(new Person("张三", "男", 20, 1500));
            persons1.Add(new Person("王成", "男", 32, 3200));
            persons1.Add(new Person("李丽", "女", 19, 1700));
            persons1.Add(new Person("何英", "女", 35, 3600));
            persons1.Add(new Person("何英", "女", 18, 1600));
            dataGridView1.DataSource = persons1;  

        }  

        private void button1_Click(object sender, EventArgs e)
        {
            //******* 对集合按Name属于进行分组GroupBy查询 ********
            //结果中包括的字段：
            //1、分组的关键字：Name = g.Key
            //2、每个分组的数量：count = g.Count()
            //3、每个分组的年龄总和：ageC = g.Sum(item => item.Age)
            //4、每个分组的收入总和：moneyC = g.Sum(item => item.Money)  

            //写法1：Lambda 表达式写法（推荐）
            var ls = persons1.GroupBy(a => a.Name).Select(g => (new { name = g.Key, count = g.Count(), ageC = g.Sum(item => item.Age), moneyC = g.Sum(item => item.Money) }));
            //写法2：类SQL语言写法 最终编译器会把它转化为lamda表达式
            var ls2 = from ps in persons1
                     group ps by ps.Name
                         into g
                         select new { name = g.Key, count = g.Count(), ageC = g.Sum(item => item.Age), moneyC = g.Sum(item => item.Money) };  

            dataGridView1.DataSource = ls.ToList();
           //dataGridView1.DataSource = ls2.ToList();
        }
    }  

    /// <summary>
    /// 手动设计一个Person类。用于放到List泛型中
    /// </summary>
    public class Person
    {
        public string Name { get; set; }
        public int Age  { get;private set; }
        public string Sex { get; set; }
        public int Money { get; set; }  

        public Person(string name, string sex, int age, int money)
        {
            Name = name;
            Age = age;
            Sex = sex;
            Money = money;
        }
    }

  // 自己 项目中使用到
  //List<view_teaching_class_student_relation> listSingleStudentCourseInfo = new List<view_teaching_class_student_relation>();
//listSingleStudentCourseInfo 已赋值，赋值省略
 // results list 包含group_course_name group_course_id，count 三个属性，倒序
   var results = listAllStudentCourseInfo.GroupBy(x => new { x.group_course_name,x.group_course_id }).Select(g => (new { course_name = g.Key,  count = g.Count() })).OrderByDescending(t=>t.count).ToList();
    public class view_teaching_class_student_relation
    {
       public string student_id { get; set; }
       public string class_id { get; set; }

       public string group_course_id { get; set; }
       public string group_course_name { get; set; }

       public string realname_cn { get; set; }

    }