怎么在c#中使用Linq查詢語句？相信很多沒有經驗的人對此束手無策，為此本文總結了問題出現的原因和解決方法，通過這篇文章希望你能解決這個問題。
Lambda表達式簡介：
/*Lambda表達式:Lambda 表達式是一種可用于創建委托或表達式目錄樹類型的匿名函數
表達式位于 => 運算符右側的 lambda 表達式稱為“表達式 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
}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表達式
例子：
/// <summary>
/// 學生結構體
/// </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>
/// 分數
/// </summary>
public float score;
}
/// <summary>
/// linq
/// 基本查詢
/// </summary>
static void Linq1()
{
#region 構造查詢數據
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個學生中選擇出不及格的人員名單并按分數降序排列
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("分數："+st.score);
}
Console.ReadKey();
}linq查詢為延遲查詢，只需構造一次查詢語句，可以多次使用。
例子：
/// <summary>
/// Linq
/// 延遲查詢
/// linq查詢為延遲查詢，只需構造一次查詢語句，可以多次使用
/// </summary>
static void Linq2()
{
#region 構造查詢數據
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("第一次查詢結果：");
foreach (string st in query)
{
Console.WriteLine( st);
}
Console.WriteLine("第二次查詢結果：");
lists.Add("Jone");
lists.Add("Jimi");
lists.Add("Johu");
foreach (string st in query)
{
Console.WriteLine(st);
}
Console.ReadKey();
/*
輸出結果：
* 第一次：Jack
* 第二次：Jack Jone Jimi Johu
*/
}利用OfType方法可以把特定類型數據篩選出來。
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();
/*
輸出結果：
* one two three
*/
}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 + "，分數" + +s.score; /*查詢出所有姓趙的學生的班級、姓名、分數信息，并按分數由高到低排序*/
例子：
/// <summary>
/// linq復合from字句
/// </summary>
static void Linq4()
{
#region 構造查詢數據
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 + "，分數" + +s.score;
/*查詢出所有姓趙的學生的班級、姓名、分數信息，并按分數由高到低排序*/
foreach (var t in query)
{
Console.WriteLine(t);
}
Console.ReadKey();
}語句：
//參數越靠前，優先級越高 //先按score排序，當分數相同時再按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 構造查詢數據
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
//參數越靠前，優先級越高
//先按score排序，當分數相同時再按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("分數：" + st.score);
}
Console.ReadKey();
}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：{1}個", o.country, o.count);
}
Console.ReadKey();
}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();
}.Net4.0新增，可對兩個相關的序列進行合并。
語句：
/*若合并兩項項數不同，則在達到較小集合的末尾時停止*/
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，可對兩個相關的序列進行合并
/// </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 };
/*若合并兩項項數不同，則在達到較小集合的末尾時停止*/
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();
}通過Take（）和Skip（）實現只顯示部分查詢結果。
語句：
//通過skip跳過指定數量的元素，再通過take提取固定長度元素，可實現分頁 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（分頁）-通過Take（）和Skip（）實現只顯示部分查詢結果
/// </summary>
static void Linq9()
{
#region 構造查詢數據
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跳過指定數量的元素，再通過take提取固定長度元素，可實現分頁
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();
}并行查詢可以分解查詢的工作，使其分布在多個線程上。當pc擁有多個cpu時，可以看到并行查詢帶來的改進效果。并行LINQ適用于大型的集合查詢，并擁有一定的優勢。使用
System.Collections.Concurrent.Partitioner.Create
可以手動創建分區器。
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表達式）
例子：
/// <summary>
/// 并行linq
/// </summary>
static void Linq11()
{
Console.WriteLine("開始構造大數組...");
//構造大數組
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("開始計算...");
var st = System.DateTime.Now;
var sum = (from x in data where x > 20 select x).Sum();//常規linq-耗時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0.6620s
//var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或并行查詢----x3 => x3 > 20（Lambda表達式）
var st3 = System.DateTime.Now;
/*Partitioner.Create
手動創建分區器
* Create具有多個重載，可依據需求進行分區
*/
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linq耗時：{0}s",dt1.TotalSeconds.ToString());
Console.WriteLine("并行linq耗時：{0}s", dt2.TotalSeconds.ToString());
Console.ReadKey();
}輸出結果：
可以粗魯的認為并行linq對于大集合的查詢是優勢比較明顯的。
對于并行ling而言，可以設置可以利用System.Threading.CancellationTokenSource設置取消操作。
語法：
//通過.WithCancellation(cts.Token)設置 var sum = (from x in data.AsParallel().WithCancellation(cts.Token) where x < 20 select x).Average();
例子：
/// <summary>
/// 取消長時間運行的并行linq查詢
/// </summary>
static void Linq12()
{
//構造大數組
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("計算開始...");
Console.WriteLine("取消請按n！");
var rk = Console.Read();
if(rk=='n'||rk=='N')
{
cts.Cancel();
}
}看完上述內容，你們掌握怎么在c#中使用Linq查詢語句的方法了嗎？如果還想學到更多技能或想了解更多相關內容，歡迎關注億速云行業資訊頻道，感謝各位的閱讀！
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