c# Linq查询详解
作者：DullFish
这篇文章主要介绍了c# Linq查询的相关资料，帮助大家更好的理解和学习使用c#，感兴趣的朋友可以了解下
c#提供的ling查询极大的遍历了集合的查询过程，且使用简单方便，非常的有用。
下面将分别用简单的例子说明：ling基本查询、延迟查询属性、类型筛选、复合from字句、多级排序、分组查询、联合查询、合并、分页、聚合操作符、并行linq、取消长时间运行的并行ling查询。
Lambda表达式简介：
/*Lambda表达式:Lambda 表达式是一种可用于创建委托或表达式目录树类型的匿名函数
表达式位于 => 运算符右侧的 lambda 表达式称为“表达式 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
}
基本查询
语句：
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表达式
例子：
/// <summary>
/// 学生结构体
/// </summary>
struct Student
{
/// <summary>
/// 姓名
/// </summary>
public string name;
/// <summary>
/// 年龄
/// </summary>
public int age;
/// <summary>
/// 班号
/// </summary>
public int grade;
/// <summary>
/// 分数
/// </summary>
public float score;
}
/// <summary>
/// linq
/// 基本查询
/// </summary>
static void Linq1()
{
#region 构造查询数据
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 = "孙xxx"; break;
case 4: name = "钱xxx"; break;
default: name = "赵xxx"; break;
}
Student psdt = new Student();
psdt.name = name;
psdt.age = age;
psdt.grade = gde;
psdt.score = sc;
listStudents.Add(psdt);
}
#endregion
//从50个学生中选择出不及格的人员名单并按分数降序排列
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表达式
//orderby升序即从小到大，orderby r.score descending降序即从大到小
Console.WriteLine("不及格的人员名单：");
foreach (Student st in query)
{
Console.WriteLine("***************");
Console.WriteLine("姓名："+st.name);
Console.WriteLine("班级："+st.grade);
Console.WriteLine("年龄："+st.age);
Console.WriteLine("分数："+st.score);
}
Console.ReadKey();
}
延迟查询属性
linq查询为延迟查询，只需构造一次查询语句，可以多次使用。
例子：
/// <summary>
/// Linq
/// 延迟查询
/// linq查询为延迟查询，只需构造一次查询语句，可以多次使用
/// </summary>
static void Linq2()
{
#region 构造查询数据
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("第一次查询结果：");
foreach (string st in query)
{
Console.WriteLine( st);
}
Console.WriteLine("第二次查询结果：");
lists.Add("Jone");
lists.Add("Jimi");
lists.Add("Johu");
foreach (string st in query)
{
Console.WriteLine(st);
}
Console.ReadKey();
/*
输出结果：
* 第一次：Jack
* 第二次：Jack Jone Jimi Johu
*/
}
类型筛选
利用OfType方法可以把特定类型数据筛选出来。
例子
/// <summary>
/// Linq类型筛选-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();
/*
输出结果：
* one two three
*/
}
复合from子句
语句：
var query = from s in listStudents
from n in s.name
where n == '赵' orderby s.score descending
select s.grade + "班-" + s.name + "，分数" + +s.score;
/*查询出所有姓赵的学生的班级、姓名、分数信息，并按分数由高到低排序*/
例子：
/// <summary>
/// linq复合from字句
/// </summary>
static void Linq4()
{
#region 构造查询数据
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 = "孙xxx"; break;
case 4: name = "钱xxx"; break;
default: name = "赵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 == '赵' orderby s.score descending
select s.grade + "班-" + s.name + "，分数" + +s.score;
/*查询出所有姓赵的学生的班级、姓名、分数信息，并按分数由高到低排序*/
foreach (var t in query)
{
Console.WriteLine(t);
}
Console.ReadKey();
}
多级排序
语句：
//参数越靠前，优先级越高
//先按score排序，当分数相同时再按grade排序...依次类推
var query = from s in listStudents orderby s.score, s.grade, s.age, s.name select s;
例子：
/// <summary>
/// 多级排序
/// </summary>
static void Linq5()
{
#region 构造查询数据
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 = "孙xxx"; break;
case 4: name = "钱xxx"; break;
default: name = "赵xxx"; break;
}
Student psdt = new Student();
psdt.name = name;
psdt.age = age;
psdt.grade = gde;
psdt.score = sc;
listStudents.Add(psdt);
}
#endregion
//参数越靠前，优先级越高
//先按score排序，当分数相同时再按grade排序...依次类推
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("班级：" + st.grade);
Console.WriteLine("年龄：" + st.age);
Console.WriteLine("分数：" + st.score);
}
Console.ReadKey();
}
分组
语句：
//按国家分组，并选出大于2的组，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>
/// 分组
/// </summary>
static void Linq6()
{
List<Champions> listChampion = new List<Champions>();
listChampion.Add(new Champions() { name = "张**", country = "中国" });
listChampion.Add(new Champions() { name = "赵**", country = "中国" });
listChampion.Add(new Champions() { name = "李**", country = "中国" });
listChampion.Add(new Champions() { name = "李**", country = "中国" });
listChampion.Add(new Champions() { name = "Peter", country = "美国" });
listChampion.Add(new Champions() { name = "Hune", country = "美国" });
listChampion.Add(new Champions() { name = "Hune", country = "美国" });
listChampion.Add(new Champions() { name = "Jack", country = "俄罗斯" });
listChampion.Add(new Champions() { name = "Jack", country = "俄罗斯" });
listChampion.Add(new Champions() { name = "Jimi", country = "英国" });
//按国家分组，并选出大于2的组，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("国家：{0}，冠军数：{1}个", o.country, o.count);
}
Console.ReadKey();
}
联合查询-join
语句：
//查询出集合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>
/// 联合查询-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;
//查询出集合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，可对两个相关的序列进行合并。
语句：
/*若合并两项项数不同，则在达到较小集合的末尾时停止*/
var qZip = qSc.Zip(qSt, (first, second) =>string.Format("Year:{0},Name:{1}.", first.Year,second.Name));//返回值qZip为字符串集合
例子：
/// <summary>
/// 合并-zip()-.Net4.0新增，可对两个相关的序列进行合并
/// </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 };
/*若合并两项项数不同，则在达到较小集合的末尾时停止*/
var qZip = qSc.Zip(qSt, (first, second) =>string.Format("Year:{0},Name:{1}.", first.Year,second.Name));//返回值qZip为字符串集合
foreach (var ite in qZip)
{
Console.WriteLine(ite);
}
Console.ReadKey();
}
分区（分页）
通过Take（）和Skip（）实现只显示部分查询结果。
语句：
//通过skip跳过指定数量的元素，再通过take提取固定长度元素，可实现分页
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>
/// 分区（分页）-通过Take（）和Skip（）实现只显示部分查询结果
/// </summary>
static void Linq9()
{
#region 构造查询数据
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 = "孙xxx"; break;
case 4: name = "钱xxx"; break;
default: name = "赵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}页", i);
//通过skip跳过指定数量的元素，再通过take提取固定长度元素，可实现分页
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linq
并行查询可以分解查询的工作，使其分布在多个线程上。当pc拥有多个cpu时，可以看到并行查询带来的改进效果。并行LINQ适用于大型的集合查询，并拥有一定的优势。使用
System.Collections.Concurrent.Partitioner.Create
可以手动创建分区器。
语法：
var sum2 = (from x2 in data.AsParallel() where x2 > 20 select x2).Sum();//并行查询 //var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或并行查询（Lambda表达式）
例子：
/// <summary>
/// 并行linq
/// </summary>
static void Linq11()
{
Console.WriteLine("开始构造大数组...");
//构造大数组
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("开始计算...");
var st = System.DateTime.Now;
var sum = (from x in data where x > 20 select x).Sum();//常规linq-耗时1.8641s
var st2 = System.DateTime.Now;
var sum2 = (from x2 in data.AsParallel() where x2 > 20 select x2).Sum();//并行查询-耗时0.6620s
//var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或并行查询----x3 => x3 > 20（Lambda表达式）
var st3 = System.DateTime.Now;
/*Partitioner.Create
手动创建分区器
* Create具有多个重载，可依据需求进行分区
*/
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("常规linq耗时：{0}s",dt1.TotalSeconds.ToString());
Console.WriteLine("并行linq耗时：{0}s", dt2.TotalSeconds.ToString());
Console.ReadKey();
}
输出结果：
可以粗鲁的认为并行linq对于大集合的查询是优势比较明显的。
取消长时间运行的并行linq查询
对于并行ling而言，可以设置可以利用System.Threading.CancellationTokenSource设置取消操作。
语法：
//通过.WithCancellation(cts.Token)设置
var sum = (from x in data.AsParallel().WithCancellation(cts.Token) where x < 20 select x).Average();
例子：
/// <summary>
/// 取消长时间运行的并行linq查询
/// </summary>
static void Linq12()
{
//构造大数组
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
{
//通过.WithCancellation(cts.Token)设置
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("计算开始...");
Console.WriteLine("取消请按n！");
var rk = Console.Read();
if(rk=='n'||rk=='N')
{
cts.Cancel();
}
}
至此，对于LINQ已经有了一个基本的了解。
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