這一節大家共同學習下LINQ的基本用法，主要包括LINQ的分組，排序，和內外連接。
1.分組
?? 基本語法:? group element by key
????????????????????? element 表示查詢結果返回的元素，key表示分組條件。group子句返回的類型為IGrouping<TKey,TElement>的查詢結果。
????????????????????? IGrouping<TKey,TElement>可以看成一個hashtable內部嵌套一個list列表的數據結果. 其中TElement就是一個list
我們看下面的一個事例，下面這個實例演示根據性別對用戶進行分組
代碼
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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;}}
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privatevoid button1_Click(object sender, EventArgs e)? ??????? {? ??????????? User[] users = {?? ?????????????????????????? new User("劉德華","男",40),? ?????????????????????????? new User("張學友","男",30),? ?????????????????????????? new User("張曼玉","女",35),? ?????????????????????????? new User("楊冪","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("劉德華","男",40),new User("張學友","男",30),new User("張曼玉","女",35),new User("楊冪","女",53)};var query1 =from var1 in usersgroup 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;}
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效果圖
以上是簡單的分組，如果需要對分組結果進行排序，需要把分組結果保存到一個臨時變量中
語法： group element by key into? temp
?? 以下是按年齡從小到大的排序：
? 代碼
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User[] users = {?? ????????????????????????? new User("劉德華","男",40),? ????????????????????????? new User("張學友","男",30),? ????????????????????????? new User("張曼玉","女",35),? ????????????????????????? new User("楊冪","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("劉德華","男",40),new User("張學友","男",30),new User("張曼玉","女",35),new User("楊冪","女",53)};var query1 =from var1 in usersgroup var1 by var1.Age into temporderby temp.Key descendingselect 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;
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?? 效果圖
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2.排序
??? 在linq中排序使用orderby
??? 語法 orderby elment? descending||ascending
????? ascending表示升序，descending表示降序
??? 下面這個DEMO演示按年齡從大到小排序
??? 代碼：
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[csharp] view plaincopyprint?
User[] users = {?? ?????????????????????????? new User("劉德華","男",40),? ?????????????????????????? new User("張學友","男",30),? ?????????????????????????? new User("張曼玉","女",35),? ?????????????????????????? new User("楊冪","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("劉德華","男",40),new User("張學友","男",30),new User("張曼玉","女",35),new User("楊冪","女",53)};var query1 =from var1 in usersorderby var1.Age descendingselect var1;string str = "";foreach (var item in query1){str += item.UserName + "--" + item.Age + "\r\n";}textBox1.Text = str;
? 效果圖
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3.內連接
??? 內連接查詢就是根據一定的條件查詢兩個數據源中都存在的元素
?? 語法： join elment? in datasource on re1 equals re2
?? 以下代碼查找兩個數組中都存在的元素。
? 代碼：
[csharp] view plaincopyprint?
privatevoid button3_Click(object sender, EventArgs e)? ??????? {? ??????????? ? ??????????? 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){//查詢兩個數組中都存在的元素int[] arrOne = { 10,20,30,6,8};int[] arrTwo = { 10,4,6,8,90,3};var query = from re1 in arrOnejoin re2 in arrTwo on re1 equals re2select new { re1 = re1, re2 = re2 };foreach (var item in query){Console.WriteLine(item);}}
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? 查詢結果：
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{ re1 = 10, re2 = 10 }? { re1 = 6, re2 = 6 }? { re1 = 8, re2 = 8 }? { re1 = 10, re2 = 10 }
{ re1 = 6, re2 = 6 }
{ re1 = 8, re2 = 8 }
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4.左連接
左連接和SQL中的左連接類似，返回第一個結果集中的所有元素。
下面的代碼演示使用左連接，左連接要用到defaultempty()方法，此方法從列表中獲取指定元素，如果列表為空，返回默認值
代碼
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privatevoid button4_Click(object sender, EventArgs e)? ????? {? ????????? ? ????????? 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){//左連接int[] arrOne = { 10, 20, 30, 6, 8 };int[] arrTwo = { 10, 4, 6, 8, 90, 3 };var query = from re1 in arrOnejoin re2 in arrTwo on re1 equals re2 into temgrpfrom grp in temgrp.DefaultIfEmpty()select new { re1 = re1, re2 = grp };foreach (var item in query){Console.WriteLine(item);}}
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查詢結果
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{ re1 = 10, re2 = 10 }? { re1 = 20, re2 = 0 }? { re1 = 30, re2 = 0 }? { re1 = 6, re2 = 6 }? { re1 = 8, re2 = 8 }?
總結
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