这一节大家共同学习下LINQ的基本用法，主要包括LINQ的分组，排序，和内外连接。
1.分组
基本语法: group element by key
element 表示查询结果返回的元素，key表示分组条件。group子句返回的类型为IGrouping<TKey,TElement>的查询结果。
IGrouping<TKey,TElement>可以看成一个hashtable内部嵌套一个list列表的数据结果. 其中TElement就是一个list
我们看下面的一个事例，下面这个实例演示根据性别对用户进行分组
代码
- publicclass User
- {
- public User(string userName,string sex,int age)
- {
- UserName = userName;
- Sex = sex;
- Age = age;
- }
- publicstring UserName
- {
- get;
- set;
- }
- publicstring Sex
- {
- get;
- set;
- }
- publicint Age
- {
- get;
- set;
- }
- }
public class User { public User(string userName,string sex,int age) { UserName = userName; Sex = sex; Age = age; } public string UserName { get; set; } public string Sex { get; set; } public int Age { get; set; } }
- privatevoid button1_Click(object sender, EventArgs e)
- {
- User[] users = {
- new User("刘德华","男",40),
- new User("张学友","男",30),
- new User("张曼玉","女",35),
- new User("杨幂","女",53)
- };
- var query1 =
- from var1 in users
- group var1 by var1.Sex;
- string str = "";
- foreach (var item in query1)
- {
- str += item.Key + "\r\n";
- foreach (var user in item)
- {
- str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "\r\n";
- }
- }
- textBox1.Text = str;
- }
private void button1_Click(object sender, EventArgs e) { User[] users = { new User("刘德华","男",40), new User("张学友","男",30), new User("张曼玉","女",35), new User("杨幂","女",53) }; var query1 = from var1 in users group var1 by var1.Sex; string str = ""; foreach (var item in query1) { str += item.Key + "\r\n"; foreach (var user in item) { str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "\r\n"; } } textBox1.Text = str; }
效果图
以上是简单的分组，如果需要对分组结果进行排序，需要把分组结果保存到一个临时变量中
语法： group element by key into temp
以下是按年龄从小到大的排序：
代码
- User[] users = {
- new User("刘德华","男",40),
- new User("张学友","男",30),
- new User("张曼玉","女",35),
- new User("杨幂","女",53)
- };
- var query1 =
- from var1 in users
- group var1 by var1.Age into temp
- orderby temp.Key descending
- select temp;
- string str = "";
- foreach (var item in query1)
- {
- str += item.Key + "\r\n";
- foreach (var user in item)
- {
- str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "\r\n";
- }
- }
- textBox1.Text = str;
User[] users = { new User("刘德华","男",40), new User("张学友","男",30), new User("张曼玉","女",35), new User("杨幂","女",53) }; var query1 = from var1 in users group var1 by var1.Age into temp orderby temp.Key descending select temp; string str = ""; foreach (var item in query1) { str += item.Key + "\r\n"; foreach (var user in item) { str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "\r\n"; } } textBox1.Text = str;
效果图
2.排序
在linq中排序使用orderby
语法 orderby elment descending||ascending
ascending表示升序，descending表示降序
下面这个DEMO演示按年龄从大到小排序
代码：
- User[] users = {
- new User("刘德华","男",40),
- new User("张学友","男",30),
- new User("张曼玉","女",35),
- new User("杨幂","女",53)
- };
- var query1 =
- from var1 in users
- orderby var1.Age descending
- select var1;
- string str = "";
- foreach (var item in query1)
- {
- str += item.UserName + "--" + item.Age + "\r\n";
- }
- textBox1.Text = str;
User[] users = { new User("刘德华","男",40), new User("张学友","男",30), new User("张曼玉","女",35), new User("杨幂","女",53) }; var query1 = from var1 in users orderby var1.Age descending select var1; string str = ""; foreach (var item in query1) { str += item.UserName + "--" + item.Age + "\r\n"; } textBox1.Text = str; 效果图
3.内连接
内连接查询就是根据一定的条件查询两个数据源中都存在的元素
语法： join elment in datasource on re1 equals re2
以下代码查找两个数组中都存在的元素。
代码：
- privatevoid button3_Click(object sender, EventArgs e)
- {
- //查询两个数组中都存在的元素
- int[] arrOne = { 10,20,30,6,8};
- int[] arrTwo = { 10,4,6,8,90,3};
- var query = from re1 in arrOne
- join re2 in arrTwo on re1 equals re2
- select new { re1 = re1, re2 = re2 };
- foreach (var item in query)
- {
- Console.WriteLine(item);
- }
- }
private void button3_Click(object sender, EventArgs e) { //查询两个数组中都存在的元素 int[] arrOne = { 10,20,30,6,8}; int[] arrTwo = { 10,4,6,8,90,3}; var query = from re1 in arrOne join re2 in arrTwo on re1 equals re2 select new { re1 = re1, re2 = re2 }; foreach (var item in query) { Console.WriteLine(item); } }
查询结果：
- { re1 = 10, re2 = 10 }
- { re1 = 6, re2 = 6 }
- { re1 = 8, re2 = 8 }
{ re1 = 10, re2 = 10 }{ re1 = 6, re2 = 6 }{ re1 = 8, re2 = 8 }
4.左连接
左连接和SQL中的左连接类似，返回第一个结果集中的所有元素。
下面的代码演示使用左连接，左连接要用到defaultempty()方法，此方法从列表中获取指定元素，如果列表为空，返回默认值
代码
- privatevoid button4_Click(object sender, EventArgs e)
- {
- //左连接
- int[] arrOne = { 10, 20, 30, 6, 8 };
- int[] arrTwo = { 10, 4, 6, 8, 90, 3 };
- var query = from re1 in arrOne
- join re2 in arrTwo on re1 equals re2 into temgrp
- from grp in temgrp.DefaultIfEmpty()
- select new { re1 = re1, re2 = grp };
- foreach (var item in query)
- {
- Console.WriteLine(item);
- }
- }
private void button4_Click(object sender, EventArgs e) { //左连接 int[] arrOne = { 10, 20, 30, 6, 8 }; int[] arrTwo = { 10, 4, 6, 8, 90, 3 }; var query = from re1 in arrOne join re2 in arrTwo on re1 equals re2 into temgrp from grp in temgrp.DefaultIfEmpty() select new { re1 = re1, re2 = grp }; foreach (var item in query) { Console.WriteLine(item); } }
查询结果
- { re1 = 10, re2 = 10 }
- { re1 = 20, re2 = 0 }
- { re1 = 30, re2 = 0 }
- { re1 = 6, re2 = 6 }
- { re1 = 8, re2 = 8 }