linq查詢表達(dá)式的基本語法很容易掌握，它使用c#常見的語言構(gòu)造，從外觀上看，和我們常用的sql類似，并且查詢表達(dá)式中的變量可以用匿名類型，所以在很多情況下，不需要指定變量類型就可以構(gòu)建linq表達(dá)式。
linq的數(shù)據(jù)源可以是數(shù)據(jù)庫對象或是xml流等，也可以使實(shí)現(xiàn)了ienumerable或者泛型ienumberable<t>接口的集合對象。
linq的基本語法包含如下的8個上下文關(guān)鍵字，這些關(guān)鍵字和具體的說明如下：
| 關(guān)鍵字 |
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| from | 指定范圍變量和數(shù)據(jù)源 |
| where | 根據(jù)bool表達(dá)式從數(shù)據(jù)源中篩選數(shù)據(jù) |
| select | 指定查詢結(jié)果中的元素所具有的類型或表現(xiàn)形式 |
| group | 對查詢結(jié)果按照鍵值進(jìn)行分組(igrouping<tkey,telement>) |
| into | 提供一個標(biāo)識符，它可以充當(dāng)對join、group或select子句結(jié)果的引用 |
| orderby | 對查詢出的元素進(jìn)行排序(ascending/descending) |
| join | 按照兩個指定匹配條件來equals連接兩個數(shù)據(jù)源 |
| let | 產(chǎn)生一個用于存儲查詢表達(dá)式中的子表達(dá)式查詢結(jié)果的范圍變量 |
下面依此總結(jié)這8個關(guān)鍵字的常用查詢語句寫法。
1.from子句
如果要寫一個linq表達(dá)式，就必須是以from子句開頭。個人覺得from子句中需要注意的地方就是多個from子句的書寫。
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using system;using system.collections.generic;using system.linq;using system.text;using system.collections;namespace linq{ /// <summary> /// linq,重點(diǎn)是感悟from子句中的查詢變量的靈活 /// </summary> class program { static void main(string[] args) { //1單個form子句 string[] values = { "linq學(xué)習(xí)","linq基本語句","from子句","單個from子句"}; var value = from v in values where v.indexof("linq") > -1 select new { v, v.length }; foreach (var n in value) { console.writeline("{0},{1}",n.v,n.length ); } console.readkey(false); //2使用linq查詢arraylist arraylist glist = new arraylist(); glist.add(new gustinfo { name="debuglzq", age=26, tel="88888888"}); glist.add(new gustinfo { name="博客園",age=6, tel ="666666"}); glist.add(new gustinfo { name = "m&msoft", age =9, tel = "55555" }); var query = from gustinfo gust in glist where gust.age > 9 select gust;//范圍變量gust制定了數(shù)據(jù)類型 foreach (gustinfo g in query) { console.writeline("{0} 年齡:{1} 電話:{2}",g.name,g.age,g.tel ); } console.readkey(false); //3復(fù)合from子句 list<gustinfo2> glist2 = new list<gustinfo2>() { new gustinfo2{ name="debuglzq",age=26,teltable=new list<string>(){"8888888","138******"}}, new gustinfo2{ name="博客園",age=6,teltable =new list<string>(){"666666","138******"}}, new gustinfo2{ name="m&msoft",age=9,teltable=new list<string>(){"55555","138******"}} }; //gust、tel都是查詢變量,作用域?yàn)楫?dāng)前查詢語句！??！ var query2 = from gust in glist2 from tel in gust.teltable where tel.indexof("5555") > -1 select gust; foreach (var g in query2) { console.writeline("{0} 年齡{1}",g.name,g.age ); foreach (var t in g.teltable) { console.writeline("電話：{0}",t); } } console.readkey(false); //4多個from子句 var query3 = from gustinfo gust in glist where gust.age > 6 from gustinfo2 gust2 in glist2 where gust2.age> 9 select new { gust, gust2 };//查詢結(jié)果定制 foreach (var g in query3) { console.writeline("{0} {1}", g.gust.name, g.gust2.name); } console.readkey(false); } }} |
程序的運(yùn)行結(jié)果如下：
程序中列舉了from子句的用法示例，注意復(fù)合from子句和多個from子句的書寫，同時(shí)需要理解范圍變量和數(shù)據(jù)源變量這兩個概念。
2.where子句
where子句，它是linq表達(dá)式的元素篩選機(jī)制，除了開始和結(jié)束的位置，它幾乎可以出現(xiàn)在linq表達(dá)式的任意位置上。
在一個linq表達(dá)式中，可以有where子句，也可以沒有；可以有一個，也可以有多個；多個where子句之間的邏輯關(guān)系相當(dāng)于邏輯“與”，每個where子句可以包含1個或多個bool邏輯表達(dá)式，這些條件成為謂詞，謂詞邏輯之間用的是“&&”“||”等而不是sql中的and 、or。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_where{ /// <summary> /// linq where子句 /// </summary> class program { static void main(string[] args) { //1常見的where語句 list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq", age=26,tel="88888888"}, new gustinfo(){ name="cnblogs",age=6,tel="666666"}, new gustinfo(){ name="m&msoft",age=9,tel="55555"} }; var query = from gust in glist where (gust.name.length > 7 || gust.name.substring(0, 1) == "m") && gust.age > 9 select new { gust.name, gust.age }; foreach (var g in query) { console.writeline("{0},{1}", g.name, g.age); } console.readkey(false); //2.在where子句中使用自定義函數(shù) var query2 = from gustinfo gust in glist where gust.name.length > 5 && check(gust.name) select gust; foreach (var g in query2) { console.writeline("{0},{1},{2}", g.name, g.age, g.tel); } console.readkey(false); //3.動態(tài)謂詞的篩選 //定義動態(tài)謂詞數(shù)組,在實(shí)際開發(fā)中可以動態(tài)獲得 string[] names = { "sb","xxx","***","@@@","一些敏感詞"}; var query3 = from gustinfo guest in glist where !names.contains(guest.name) select guest; foreach (var q in query3) { console.writeline("{0} 年齡：{1},電話：{2}",q.name,q.age,q.tel ); } console.readkey(false); } //自定義函數(shù) static bool check(string name) { if (name.substring(0, 1) == "n") return false; return true; } }} |
需要注意一些常用的where子句的寫法。程序的運(yùn)行結(jié)果如下：
3.select子句
在select子句上可以非常靈活的處理查詢到的元素，然后再把結(jié)果返回。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_select{ /// <summary> /// linq select /// 在select子句上，可以非常靈活的處理查詢到的元素，然后再把結(jié)果返回 /// </summary> class mygustinfo { public string name { get; set; } public int age { get; set; } } class program { static void main(string[] args) { list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq", age=25, tel="88888888"}, new gustinfo(){ name="cnblogs", age=6, tel="666666"}, new gustinfo(){ name="m&m", age=9, tel="55555"} }; var query = from gust in glist where gust.age >= 9 && gust.age <= 30 select gust.name.replace("&", "mm");//select子句靈活應(yīng)用 var query2 = from gust in glist where gust.age >= 9 && gust.age <= 30 select myproc(gust.name); var query3 = from gust in glist where gust.age >= 9 && gust.age <= 30 select new { gust.name,gust.age}; var query4 = from gust in glist where gust.age >= 9 && gust.age <= 30 select new mygustinfo { name=gust.name+"my", age=gust.age+1};//對查詢結(jié)果進(jìn)行投影 foreach (var v in query) { console.writeline(v); } foreach (var v in query2) { console.writeline(v); } foreach (var v in query3) { console.writeline(v.name+v.age ); } foreach (var v in query4) { console.writeline(v.name+v.age ); } console.readkey(false); } static string myproc(string s) { return s + "better"; } }} |
程序的運(yùn)行結(jié)果如下：
4.group子句
根據(jù)語法的規(guī)定，linq表達(dá)式必須以from子句開頭，以select或group子句結(jié)束，所以除了使用select來返回結(jié)果外，也可以使用group子句來返回元素分組后的結(jié)果。
group子句返回的是一個基于igrouping<tkey,telement>泛型接口的對象序列。
語法和sql的group有點(diǎn)區(qū)別，不注意的話可能會寫錯。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_group{ /// <summary> /// linq group子句 /// </summary> class program { static void main(string[] args) { list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq",age=26,tel="187********"}, new gustinfo(){ name="sarah",age=25,tel="159********"}, new gustinfo(){ name="jerry",age=35,tel="135********"}, new gustinfo(){ name="m&m",age=16,tel="136********"}, new gustinfo(){ name="debugman",age=26,tel="136********"}, new gustinfo(){ name="jerry&tom",age=19,tel="136********"}, }; var query = from guest in glist group guest by guest.name.substring(0, 1);//分組鍵key是string類型 //遍歷鍵值和鍵值所屬元素 foreach (igrouping<string, gustinfo> guestgroup in query) { console.writeline("分組鍵：{0}",guestgroup.key ); foreach (var g in guestgroup) { console.writeline("{0} 年齡：{1} 電話：{2}",g.name,g.age,g.tel ); } } console.readkey(false); console.writeline("-----------------------------------"); var query2 = from guest in glist group guest by guest.age > 20;//分組鍵key是bool類型表達(dá)式的結(jié)果 foreach (igrouping<bool, gustinfo> guestgroup in query2) { console.writeline("年齡是否大于20 分組鍵：{0}", guestgroup.key); foreach (var g in guestgroup) { console.writeline("{0} 年齡：{1} 電話：{2}", g.name, g.age, g.tel); } } console.readkey(false); } }} |
程序的運(yùn)行結(jié)果如下：
5.into子句
into子句作為一個臨時(shí)標(biāo)識符，用于group、select、join子句中充當(dāng)其結(jié)果的引用。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_into{ /// <summary> /// linq group /// </summary> class program { static void main(string[] args) { list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq",age=26,tel="187********"}, new gustinfo(){ name="sarah",age=25,tel="159********"}, new gustinfo(){ name="jerry",age=35,tel="135********"}, new gustinfo(){ name="m&m",age=16,tel="136********"}, new gustinfo(){ name="debugman",age=26,tel="136********"}, new gustinfo(){ name="jerry&tom",age=19,tel="136********"}, }; //1.into用于group子句 var query = from guest in glist group guest by guest.name.substring(0, 1) into grguest orderby grguest.key descending select grguest; var query2 = from guest in glist group guest by guest.name.substring(0, 1) into grguest orderby grguest.key ascending select grguest; //2.select 子句中的into子句 var query3 = from guest in glist select new { newname = guest.name, newage = guest.age } into newguest orderby newguest.newage select newguest; foreach (var guestgroup in query) { console.writeline("分組鍵：{0}",guestgroup.key ); foreach (var g in guestgroup) { console.writeline("{0} 電話:{1}",g.name,g.tel ); } } console.readkey(false); foreach (var newg in query3) { console.writeline("{0} 年齡：{1}",newg.newname,newg.newage ); } console.readkey(false); } }} |
程序運(yùn)行結(jié)果如下：
6.orderby子句、thenby子句
linq可以按照元素的一個或多個屬性對元素進(jìn)行排序。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_orderby{ class program { /// <summary> /// linq orderby (ascending descending) /// </summary> /// <param name="args"></param> static void main(string[] args) { list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq",age=26,tel="187********"}, new gustinfo(){ name="sarah",age=25,tel="159********"}, new gustinfo(){ name="jerry",age=35,tel="135********"}, new gustinfo(){ name="m&m",age=16,tel="136********"}, new gustinfo(){ name="debugman",age=26,tel="136********"}, new gustinfo(){ name="jerry&tom",age=19,tel="136********"}, }; //按照年齡排序 var query = from guest in glist orderby guest.age select guest; var query1 = from guest in glist orderby guest.age ascending select guest; var query2 = from guest in glist orderby guest.age descending select guest; //按照年齡進(jìn)行排序，按照名字字?jǐn)?shù)進(jìn)行次要排序 var query3 = from guest in glist orderby guest.age, guest.name.length select guest; var query4 = from guest in glist orderby guest.age descending , guest.name.length ascending select guest; var query5 = from guest in glist orderby guest.age, guest.name.length,guest.tel select guest; foreach (var guest in query2) { console.writeline("{0} 年齡：{1} 電話：{2}",guest.name,guest.age,guest.tel ); } console.readkey(false); foreach (var guest in query4) { console.writeline("{0} 年齡：{1} 電話：{2}", guest.name, guest.age, guest.tel); } console.readkey(false); } }} |
程序運(yùn)行結(jié)果如下：
7.let子句
let子句用于在linq表達(dá)式中存儲子表達(dá)式的計(jì)算結(jié)果。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_let{ /// <summary> /// linq let用來存子儲表達(dá)式的計(jì)算結(jié)果 /// </summary> class program { static void main(string[] args) { list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq",age=26,tel="187********"}, new gustinfo(){ name="sarah",age=25,tel="159********"}, new gustinfo(){ name="jerry",age=35,tel="135********"}, new gustinfo(){ name="m&m",age=16,tel="136********"}, new gustinfo(){ name="debugman",age=26,tel="136********"}, new gustinfo(){ name="jerry&tom",age=19,tel="136********"}, }; //使用let子句創(chuàng)建范圍變量g，并通過g構(gòu)建查詢表達(dá)式 var query = from guest in glist let g = guest.name.substring(0, 1) where g == "d" || g == "j" select guest; //也可以不使用let，上面的語句等效于下 var query2 = from guest in glist where guest.name.substring(0, 1) == "d" || guest.name.substring(0, 1) == "j" select guest; foreach (var g in query) { console.writeline("{0} 年齡：{1} 電話：{2}",g.name,g.age,g.tel ); } console.readkey(false); console.writeline("不使用let，等效的語句結(jié)果"); foreach (var g in query2) { console.writeline("{0} 年齡：{1} 電話：{2}", g.name, g.age, g.tel); } console.readkey(false); } }} |
程序的運(yùn)行結(jié)果如下：
8.join子句
如果一個數(shù)據(jù)源中元素的某個屬性可以跟另外一個數(shù)據(jù)源的中元素的某個屬性進(jìn)行相等比較，那么這兩個數(shù)據(jù)源可以用join子句進(jìn)行關(guān)聯(lián)。
join子句使用equals關(guān)鍵字進(jìn)行相等比較，而不是常用的雙等號。
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using system;using system.collections.generic;using system.linq;using system.text;namespace linq_join{ class program { static void main(string[] args) { //定義兩個數(shù)據(jù)源 list<gustinfo> glist = new list<gustinfo>() { new gustinfo(){ name="debuglzq",age=26,tel="187********"}, new gustinfo(){ name="sarah",age=25,tel="159********"}, new gustinfo(){ name="jerry",age=35,tel="135********"}, new gustinfo(){ name="m&m",age=16,tel="136********"}, new gustinfo(){ name="debugman",age=26,tel="136********"}, new gustinfo(){ name="jerry&tom",age=19,tel="136********"}, }; list<guesttitle> titlelist = new list<guesttitle>() { new guesttitle(){name="debuglzq",title="soft engineer"}, new guesttitle(){name="debuglzq",title="team leader"}, new guesttitle(){name="sarah",title="test engineer"}, new guesttitle(){name="jerry",title="head master"} }; //1.根據(jù)姓名進(jìn)行內(nèi)連接 var query = from guest in glist join title in titlelist on guest.name equals title.name select new { name=guest.name ,title=title.title,age=guest.age }; foreach (var g in query) { console.writeline("{0} {1} 年齡：{2}",g.name,g.title ,g.age ); } console.readkey(false); //前面的多個from實(shí)現(xiàn)相同的作用:與內(nèi)連接區(qū)別在于：這個中間的操作是叉乘獲得笛卡爾積 var query2=from guest in glist from title in titlelist where guest.name==title.name select new { name = guest.name, src="/uploads/allimg/211115/1435555948-7.jpg" />
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