c# Linq查詢詳解
c#提供的ling查詢極大的遍歷了集合的查詢過程,且使用簡單方便,非常的有用。
下面將分別用簡單的例子說明:ling基本查詢、延遲查詢屬性、類型篩選、復(fù)合from字句、多級排序、分組查詢、聯(lián)合查詢、合并、分頁、聚合操作符、并行l(wèi)inq、取消長時(shí)間運(yùn)行的并行l(wèi)ing查詢。
Lambda表達(dá)式簡介:
/*Lambda表達(dá)式:Lambda 表達(dá)式是一種可用于創(chuàng)建委托或表達(dá)式目錄樹類型的匿名函數(shù) 表達(dá)式位于 => 運(yùn)算符右側(cè)的 lambda 表達(dá)式稱為“表達(dá)式 lambda”。 * (input parameters) => expression * 示例:*/ delegate int del(int i); static void Main(string[] args) { del myDelegate = x => x * x; int j = myDelegate(5); //最后j = 25 }
基本查詢
語句:
var query = from r in listStudents where r.score < 60 orderby r.score descending select r; //var q2 = listStudents.Where(r => r.score < 60).OrderByDescending(r => r.score).Select(r => r) ;//或使用Lambda表達(dá)式
例子:
/// <summary> /// 學(xué)生結(jié)構(gòu)體 /// </summary> struct Student { /// <summary> /// 姓名 /// </summary> public string name; /// <summary> /// 年齡 /// </summary> public int age; /// <summary> /// 班號 /// </summary> public int grade; /// <summary> /// 分?jǐn)?shù) /// </summary> public float score; } /// <summary> /// linq /// 基本查詢 /// </summary> static void Linq1() { #region 構(gòu)造查詢數(shù)據(jù) List<Student> listStudents = new List<Student>(); Random pRandom = new Random(); for (int i = 1; i < 50; i++) { float sc = pRandom.Next(0, 100); int age = pRandom.Next(8, 13); int gde = pRandom.Next(1, 6); string name = ""; switch (pRandom.Next(0, 6)) { case 1: name = "周xxx"; break; case 2: name = "李xxx"; break; case 3: name = "孫xxx"; break; case 4: name = "錢xxx"; break; default: name = "趙xxx"; break; } Student psdt = new Student(); psdt.name = name; psdt.age = age; psdt.grade = gde; psdt.score = sc; listStudents.Add(psdt); } #endregion //從50個(gè)學(xué)生中選擇出不及格的人員名單并按分?jǐn)?shù)降序排列 var query = from r in listStudents where r.score < 60 orderby r.score descending select r; //var q2 = listStudents.Where(r => r.score < 60).OrderByDescending(r => r.score).Select(r => r) ;//或使用Lambda表達(dá)式 //orderby升序即從小到大,orderby r.score descending降序即從大到小 Console.WriteLine("不及格的人員名單:"); foreach (Student st in query) { Console.WriteLine("***************"); Console.WriteLine("姓名:"+st.name); Console.WriteLine("班級:"+st.grade); Console.WriteLine("年齡:"+st.age); Console.WriteLine("分?jǐn)?shù):"+st.score); } Console.ReadKey(); }
延遲查詢屬性
linq查詢?yōu)檠舆t查詢,只需構(gòu)造一次查詢語句,可以多次使用。
例子:
/// <summary> /// Linq /// 延遲查詢 /// linq查詢?yōu)檠舆t查詢,只需構(gòu)造一次查詢語句,可以多次使用 /// </summary> static void Linq2() { #region 構(gòu)造查詢數(shù)據(jù) List<string> lists = new List<string> { "Jack","Pet","Hant","Li","Kkk"}; #endregion var query = from r in lists where r.StartsWith("J") select r; Console.WriteLine("第一次查詢結(jié)果:"); foreach (string st in query) { Console.WriteLine( st); } Console.WriteLine("第二次查詢結(jié)果:"); lists.Add("Jone"); lists.Add("Jimi"); lists.Add("Johu"); foreach (string st in query) { Console.WriteLine(st); } Console.ReadKey(); /* 輸出結(jié)果: * 第一次:Jack * 第二次:Jack Jone Jimi Johu */ }
類型篩選
利用OfType方法可以把特定類型數(shù)據(jù)篩選出來。
例子
/// <summary> /// Linq類型篩選-ofType /// </summary> static void Linq3() { object[] pList = new object[] { 1,"one",2,"two",3,"three"}; var query = pList.OfType<string>(); foreach (var st in query) { Console.WriteLine(st); } Console.ReadKey(); /* 輸出結(jié)果: * one two three */ }
復(fù)合from子句
語句:
var query = from s in listStudents from n in s.name where n == '趙' orderby s.score descending select s.grade + "班-" + s.name + ",分?jǐn)?shù)" + +s.score; /*查詢出所有姓趙的學(xué)生的班級、姓名、分?jǐn)?shù)信息,并按分?jǐn)?shù)由高到低排序*/
例子:
/// <summary> /// linq復(fù)合from字句 /// </summary> static void Linq4() { #region 構(gòu)造查詢數(shù)據(jù) List<Student> listStudents = new List<Student>(); Random pRandom = new Random(); for (int i = 1; i < 50; i++) { float sc = pRandom.Next(0, 100); int age = pRandom.Next(8, 13); int gde = pRandom.Next(1, 6); string name = ""; switch (pRandom.Next(0, 6)) { case 1: name = "周xxx"; break; case 2: name = "李xxx"; break; case 3: name = "孫xxx"; break; case 4: name = "錢xxx"; break; default: name = "趙xxx"; break; } Student psdt = new Student(); psdt.name = name; psdt.age = age; psdt.grade = gde; psdt.score = sc; listStudents.Add(psdt); } #endregion var query = from s in listStudents from n in s.name where n == '趙' orderby s.score descending select s.grade + "班-" + s.name + ",分?jǐn)?shù)" + +s.score; /*查詢出所有姓趙的學(xué)生的班級、姓名、分?jǐn)?shù)信息,并按分?jǐn)?shù)由高到低排序*/ foreach (var t in query) { Console.WriteLine(t); } Console.ReadKey(); }
多級排序
語句:
//參數(shù)越靠前,優(yōu)先級越高 //先按score排序,當(dāng)分?jǐn)?shù)相同時(shí)再按grade排序...依次類推 var query = from s in listStudents orderby s.score, s.grade, s.age, s.name select s;
例子:
/// <summary> /// 多級排序 /// </summary> static void Linq5() { #region 構(gòu)造查詢數(shù)據(jù) List<Student> listStudents = new List<Student>(); Random pRandom = new Random(); for (int i = 1; i < 50; i++) { float sc = pRandom.Next(0, 100); int age = pRandom.Next(8, 13); int gde = pRandom.Next(1, 6); string name = ""; switch (pRandom.Next(0, 6)) { case 1: name = "周xxx"; break; case 2: name = "李xxx"; break; case 3: name = "孫xxx"; break; case 4: name = "錢xxx"; break; default: name = "趙xxx"; break; } Student psdt = new Student(); psdt.name = name; psdt.age = age; psdt.grade = gde; psdt.score = sc; listStudents.Add(psdt); } #endregion //參數(shù)越靠前,優(yōu)先級越高 //先按score排序,當(dāng)分?jǐn)?shù)相同時(shí)再按grade排序...依次類推 var query = from s in listStudents orderby s.score, s.grade, s.age, s.name select s; foreach (Student st in query) { Console.WriteLine("***************"); Console.WriteLine("姓名:" + st.name); Console.WriteLine("班級:" + st.grade); Console.WriteLine("年齡:" + st.age); Console.WriteLine("分?jǐn)?shù):" + st.score); } Console.ReadKey(); }
分組
語句:
//按國家分組,并選出大于2的組,形成新的集合 var query = from r in listChampion group r by r.country into g orderby g.Count() descending, g.Key where g.Count() >= 2 select new { country = g.Key, count = g.Count() };
例子:
/// <summary> /// 分組 /// </summary> static void Linq6() { List<Champions> listChampion = new List<Champions>(); listChampion.Add(new Champions() { name = "張**", country = "中國" }); listChampion.Add(new Champions() { name = "趙**", country = "中國" }); listChampion.Add(new Champions() { name = "李**", country = "中國" }); listChampion.Add(new Champions() { name = "李**", country = "中國" }); listChampion.Add(new Champions() { name = "Peter", country = "美國" }); listChampion.Add(new Champions() { name = "Hune", country = "美國" }); listChampion.Add(new Champions() { name = "Hune", country = "美國" }); listChampion.Add(new Champions() { name = "Jack", country = "俄羅斯" }); listChampion.Add(new Champions() { name = "Jack", country = "俄羅斯" }); listChampion.Add(new Champions() { name = "Jimi", country = "英國" }); //按國家分組,并選出大于2的組,形成新的集合 var query = from r in listChampion group r by r.country into g orderby g.Count() descending, g.Key where g.Count() >= 2 select new { country = g.Key, count = g.Count() }; foreach (var o in query) { Console.WriteLine("國家:{0},冠軍數(shù):{1}個(gè)", o.country, o.count); } Console.ReadKey(); }
聯(lián)合查詢-join
語句:
//查詢出集合qSt中year等于集合qSc中year的元素并形成新的集合 var qjoin = from r in qSt join c in qSc on r.year equals c.year select new { Year = r.year, stName = r.name, scName = c.name };
例子:
/// <summary> /// 聯(lián)合查詢-join /// </summary> static void Linq7() { List<s> pSt = new List<s>(); pSt.Add(new s() {year=1999,name="xxx" }); pSt.Add(new s() { year = 2000, name = "xxx" }); pSt.Add(new s() { year = 2001, name = "xxx" }); pSt.Add(new s() { year = 2010, name = "xxx" }); List<school> pSc = new List<school>(); pSc.Add(new school() { year = 1999, name = "***" }); pSc.Add(new school() { year = 2001, name = "***" }); pSc.Add(new school() { year = 2002, name = "***" }); pSc.Add(new school() { year = 2010, name = "***" }); pSc.Add(new school() { year = 2012, name = "***" }); var qSt = from r in pSt where r.year >= 2000 select r; var qSc = from r in pSc where r.year >= 2000 select r; //查詢出集合qSt中year等于集合qSc中year的元素并形成新的集合 var qjoin = from r in qSt join c in qSc on r.year equals c.year select new { Year = r.year, stName = r.name, scName = c.name }; foreach (var ite in qjoin) { Console.WriteLine(ite.Year + " " + ite.scName + " " + ite.stName); Console.WriteLine(""); } Console.ReadKey(); }
合并-zip()
.Net4.0新增,可對兩個(gè)相關(guān)的序列進(jìn)行合并。
語句:
/*若合并兩項(xiàng)項(xiàng)數(shù)不同,則在達(dá)到較小集合的末尾時(shí)停止*/ var qZip = qSc.Zip(qSt, (first, second) =>string.Format("Year:{0},Name:{1}.", first.Year,second.Name));//返回值qZip為字符串集合
例子:
/// <summary> /// 合并-zip()-.Net4.0新增,可對兩個(gè)相關(guān)的序列進(jìn)行合并 /// </summary> static void Linq8() { List<s> pSt = new List<s>(); pSt.Add(new s() { year = 1999, name = "一xxx" }); pSt.Add(new s() { year = 2000, name = "二xxx" }); pSt.Add(new s() { year = 2001, name = "三xxx" }); pSt.Add(new s() { year = 2010, name = "四xxx" }); pSt.Add(new s() { year = 2010, name = "五xxx" }); List<school> pSc = new List<school>(); pSc.Add(new school() { year = 1999, name = "1***" }); pSc.Add(new school() { year = 2012, name = "2***" }); pSc.Add(new school() { year = 2012, name = "3***" }); pSc.Add(new school() { year = 2012, name = "4***" }); pSc.Add(new school() { year = 2012, name = "5***" }); var qSt = from r in pSt where r.year >= 2000 orderby r.year select new { Year = r.year, Name = r.name }; var qSc = from r in pSc where r.year >= 2000 orderby r.year select new { Year = r.year, Name = r.name }; /*若合并兩項(xiàng)項(xiàng)數(shù)不同,則在達(dá)到較小集合的末尾時(shí)停止*/ var qZip = qSc.Zip(qSt, (first, second) =>string.Format("Year:{0},Name:{1}.", first.Year,second.Name));//返回值qZip為字符串集合 foreach (var ite in qZip) { Console.WriteLine(ite); } Console.ReadKey(); }
分區(qū)(分頁)
通過Take()和Skip()實(shí)現(xiàn)只顯示部分查詢結(jié)果。
語句:
//通過skip跳過指定數(shù)量的元素,再通過take提取固定長度元素,可實(shí)現(xiàn)分頁 var qr = (from r in listStudents orderby r.score descending select r).Skip(i * pageSize).Take(5); //var qr2 = listStudents.OrderByDescending(r => r.score).Select(r => r).Skip(i * pageSize).Take(5);//或
例子
/// <summary> /// 分區(qū)(分頁)-通過Take()和Skip()實(shí)現(xiàn)只顯示部分查詢結(jié)果 /// </summary> static void Linq9() { #region 構(gòu)造查詢數(shù)據(jù) List<Student> listStudents = new List<Student>(); Random pRandom = new Random(); for (int i = 1; i < 50; i++) { float sc = pRandom.Next(0, 100); int age = pRandom.Next(8, 13); int gde = pRandom.Next(1, 6); string name = ""; switch (pRandom.Next(0, 6)) { case 1: name = "周xxx"; break; case 2: name = "李xxx"; break; case 3: name = "孫xxx"; break; case 4: name = "錢xxx"; break; default: name = "趙xxx"; break; } Student psdt = new Student(); psdt.name = name; psdt.age = age; psdt.grade = gde; psdt.score = sc; listStudents.Add(psdt); } #endregion int pageSize = 5; int numofPage = (int)Math.Ceiling(listStudents.Count /(double)pageSize); for (int i = 0; i < numofPage; i++) { Console.WriteLine("第{0}頁", i); //通過skip跳過指定數(shù)量的元素,再通過take提取固定長度元素,可實(shí)現(xiàn)分頁 var qr = (from r in listStudents orderby r.score descending select r).Skip(i * pageSize).Take(5); //var qr2 = listStudents.OrderByDescending(r => r.score).Select(r => r).Skip(i * pageSize).Take(5);//或 foreach (var ite in qr) { Console.WriteLine(ite.name); Console.WriteLine(ite.score); } Console.WriteLine(""); } Console.ReadKey(); }
并行l(wèi)inq
并行查詢可以分解查詢的工作,使其分布在多個(gè)線程上。當(dāng)pc擁有多個(gè)cpu時(shí),可以看到并行查詢帶來的改進(jìn)效果。并行LINQ適用于大型的集合查詢,并擁有一定的優(yōu)勢。使用
System.Collections.Concurrent.Partitioner.Create
可以手動(dòng)創(chuàng)建分區(qū)器。
語法:
var sum2 = (from x2 in data.AsParallel() where x2 > 20 select x2).Sum();//并行查詢 //var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或并行查詢(Lambda表達(dá)式)
例子:
/// <summary> /// 并行l(wèi)inq /// </summary> static void Linq11() { Console.WriteLine("開始構(gòu)造大數(shù)組..."); //構(gòu)造大數(shù)組 const int count = 100000000; var data = new int[count]; var r = new Random(); for (int i = 0; i < count; i++) { data[i] = r.Next(40); } Console.WriteLine("開始計(jì)算..."); var st = System.DateTime.Now; var sum = (from x in data where x > 20 select x).Sum();//常規(guī)linq-耗時(shí)1.8641s var st2 = System.DateTime.Now; var sum2 = (from x2 in data.AsParallel() where x2 > 20 select x2).Sum();//并行查詢-耗時(shí)0.6620s //var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或并行查詢----x3 => x3 > 20(Lambda表達(dá)式) var st3 = System.DateTime.Now; /*Partitioner.Create 手動(dòng)創(chuàng)建分區(qū)器 * Create具有多個(gè)重載,可依據(jù)需求進(jìn)行分區(qū) */ var sum4 = (from c in System.Collections.Concurrent.Partitioner.Create(data, true).AsParallel() where c > 20 select c).Sum(); var dt1 = st2 - st; var dt2 = st3 - st2; Console.WriteLine("常規(guī)linq耗時(shí):{0}s",dt1.TotalSeconds.ToString()); Console.WriteLine("并行l(wèi)inq耗時(shí):{0}s", dt2.TotalSeconds.ToString()); Console.ReadKey(); }
輸出結(jié)果:
可以粗魯?shù)恼J(rèn)為并行l(wèi)inq對于大集合的查詢是優(yōu)勢比較明顯的。
取消長時(shí)間運(yùn)行的并行l(wèi)inq查詢
對于并行l(wèi)ing而言,可以設(shè)置可以利用System.Threading.CancellationTokenSource設(shè)置取消操作。
語法:
//通過.WithCancellation(cts.Token)設(shè)置 var sum = (from x in data.AsParallel().WithCancellation(cts.Token) where x < 20 select x).Average();
例子:
/// <summary> /// 取消長時(shí)間運(yùn)行的并行l(wèi)inq查詢 /// </summary> static void Linq12() { //構(gòu)造大數(shù)組 const int count = 100000000; var data = new int[count]; var r = new Random(); for (int i = 0; i < count; i++) { data[i] = r.Next(40); } var cts = new System.Threading.CancellationTokenSource(); new System.Threading.Thread(() => { try { //通過.WithCancellation(cts.Token)設(shè)置 var sum = (from x in data.AsParallel().WithCancellation(cts.Token) where x < 20 select x).Average(); Console.WriteLine("sum:{0}", sum); } catch(OperationCanceledException ex) { Console.WriteLine(ex.Message); } }).Start(); Console.WriteLine("計(jì)算開始..."); Console.WriteLine("取消請按n!"); var rk = Console.Read(); if(rk=='n'||rk=='N') { cts.Cancel(); } }
至此,對于LINQ已經(jīng)有了一個(gè)基本的了解。
以上就是c# Linq查詢詳解的詳細(xì)內(nèi)容,更多關(guān)于c# Linq查詢的資料請關(guān)注腳本之家其它相關(guān)文章!
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