语言集成查询 (LINQ) 是一系列直接将查询功能集成到 C# 语言的技术统称。
数据查询历来都表示为简单的字符串，没有编译时类型检查或 IntelliSense 支持。 此外，需要针对每种类型的数据源了解不同的查询语言：SQL 数据库、XML 文档、各种 Web 服务等。
借助 LINQ，查询成为了最高级的语言构造，就像类、方法和事件一样。 可以使用语言关键字和熟悉的运算符针对强类型化对象集合编写查询。 LINQ 系列技术提供了针对对象 (LINQ to Objects)、关系数据库 (LINQ to SQL) 和 XML (LINQ to XML) 的一致查询体验。
对于编写查询的开发者来说，LINQ 最明显的“语言集成”部分就是查询表达式。
查询表达式采用声明性查询语法编写而成。 使用查询语法，可以用最少的代码对数据源执行筛选、排序和分组操作。 可使用相同的基本查询表达式模式来查询和转换 SQL 数据库、ADO .NET 数据集、XML 文档和流以及 .NET 集合中的数据。
在 C# 中可为以下对象编写 LINQ 查询：SQL Server 数据库、XML 文档、ADO.NET 数据集以及支持 IEnumerable 或泛型 IEnumerable 接口的任何对象集合。 此外，第三方也为许多 Web 服务和其他数据库实现提供了 LINQ 支持。
实现案例using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace LinkConsole
{
class Program
{
static void Main(string[] args)
{
//-------------最基本的简单查询----------------//
List<int> numbers = new List<int>() { 1,2,3,4,5,6,7,8,9,10};
var numQuery = from num in numbers
where num % 2 == 0
select num;
foreach (var num in numQuery)
{
Console.WriteLine("{0,1}", num);
}
//-------------读取List<>中的句子----------------//
FormExpDemo2();
//-------------复合form子句----------------//
FormExpDemo();
//-------------多个from句子---------------//
FormExpDemo3();
//-------------where-------------------//
WhereExpDemo();
//-------------select------------------//
SelectDemo();
//-------------group--------------------//
GroupDemo();
//-------------into------------------------//
IntoDemo();
//--------------OrderBy--------------------//
ThenByDemo();
//--------------let----------------------//
LetDemo();
//--------------join--------------------//
JoinDemo();
Console.ReadLine();
}
public class CustomerInfo
{
public string Name { get; set; }
public int Age { get; set; }
public string Tel { get; set; }
public List<string> telTable { get; set; }
}
public static void FormExpDemo2()
{
//
List<CustomerInfo> customers = new List<CustomerInfo>
{
new CustomerInfo{ Name = "欧阳晓晓",Age = 35,Tel = "123"},
new CustomerInfo{ Name = "上官飘飘",Age = 17,Tel = "456"},
new CustomerInfo{ Name = "诸葛菲菲",Age = 23,Tel = "789"}
};
var query = from ci in customers
where ci.Age > 20
select ci;
foreach (var ci in query)
{
Console.WriteLine("姓名：{0}年龄：{1}电话：{2}", ci.Name, ci.Age, ci.Tel);
}
}
//复合from子句 // 相当于两个for循环而已
private static void FormExpDemo()
{
List<CustomerInfo> customers = new List<CustomerInfo>
{
new CustomerInfo { Name = "欧阳小小",Age= 35,telTable = new List<string> {"123","234"} },
new CustomerInfo { Name = "上官飘飘",Age= 35,telTable = new List<string> {"456","567"} },
new CustomerInfo { Name = "诸葛菲菲",Age= 35,telTable = new List<string> {"789","456"} },
};
//查询包含电话号码456的客户
var query = from ci in customers
from tel in ci.telTable
where tel.IndexOf("456") > -1
select ci;
foreach (var ci in query)
{
Console.WriteLine("姓名：{0}年龄：{1}", ci.Name, ci.Age);
foreach (var tel in ci.telTable)
{
Console.WriteLine(" 电话：{0}", tel);
}
}
}
//多个from子句，和复合子句看起来是一样的，其实不一样，一个是单个数据源中的子元素的集合，一个是对多个数据源进行查询
private static void FormExpDemo3()
{
List<CustomerInfo> customers = new List<CustomerInfo>
{
new CustomerInfo{ Name = "欧阳晓晓",Age = 35,Tel = "123"},
new CustomerInfo{ Name = "上官飘飘",Age = 77,Tel = "456"},
new CustomerInfo{ Name = "诸葛菲菲",Age = 23,Tel = "789"}
};
List<CustomerInfo> customers2 = new List<CustomerInfo>
{
new CustomerInfo{ Name = "令狐冲",Age = 25,Tel = "123"},
new CustomerInfo{ Name = "东方不败",Age = 15,Tel = "456"},
new CustomerInfo{ Name = "任盈盈",Age = 13,Tel = "789"}
};
//在customers 中寻找年龄大于20的客户
//在customenrs中寻找年龄小于30岁的客户
var query = from custo in customers
where custo.Age > 20
from custo2 in customers2
where custo2.Age < 30
select new { custo, custo2 };
foreach (var ci in query)
{
Console.WriteLine("{0},{1}", ci.custo.Name, ci.custo2.Name);//这样得到的是一个交叉联结表，有点类似于SQL中的笛卡尔沉积
}
}
//where子句查询
//where就是用来筛选元素的，除了开始和结束位置，where可以在任意位置使用，
//一个LIKQ语句中可以有where子句，也可以没有，可以有一个，也可以有多个。
//多个where子句之间的关系相当于逻辑“与”，每个子句中又可以包含多个用“谓词”链接的逻辑表达式，&&，或者||
private static void WhereExpDemo()
{
List<CustomerInfo> clist = new List<CustomerInfo>
{
new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708****"},
new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842****"},
new CustomerInfo{ Name="令狐冲", Age=23, Tel ="1380524****"}
};
//可以查询符合多个条件的人(名字是三个字或者姓令的，但年龄必须大于20)
var query = from custo in clist
where (custo.Name.Length == 3 || custo.Name.Substring(0, 1) == "令") && custo.Age > 20
select custo;//select 也可以改成，比如custo.Name。或者用一个函数，把变量传出去
foreach (var ci in query)
{
Console.WriteLine("姓名：{0}年龄：{1}电话：{2}", ci.Name, ci.Age, ci.Tel);
}
//where中使用自定义函数,查询三个字并且姓令的客户
var query2 = from custo in clist
where (custo.Name.Length == 3 && ChechName(custo.Name))
select custo;
foreach (var ci in query2)
{
Console.WriteLine("姓名：{0}年龄：{1}电话：{2}", ci.Name, ci.Age, ci.Tel);
}
}
private static bool ChechName(string name)
{
if (name.Substring(0, 1) == "令")
return true;
else
return false;
}
//select 用法举例
private static void SelectDemo()
{
List<CustomerInfo> clist = new List<CustomerInfo>
{
new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708****"},
new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842****"},
new CustomerInfo{ Name="令狐冲", Age=23, Tel ="1380524****"}
};
string[] names = { "令狐冲", "任盈盈", "杨过", "小龙女", "欧阳小夏", "欧阳晓晓" };
//查询在给定谓词数组里存在的客户
var query = from custo in clist
where custo.Age < 30
select new MyCustomerInfo { Name = custo.Name, Tel = custo.Tel };
foreach (var ci in query)
{
Console.WriteLine("姓名：{0}电话：{1}类型{2}", ci.Name, ci.Tel, ci.GetType().FullName);
}
}
public class MyCustomerInfo
{
public string Name { get; set; }
public string Tel { get; set; }
}
//-------------------Group----------------------//
static List<CustomerInfo> clist = new List<CustomerInfo>
{
new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708"},
new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842"},
new CustomerInfo{ Name="欧阳锦鹏", Age=35, Tel ="1330708"},
new CustomerInfo{ Name="上官无忌", Age=23, Tel ="1380524"}
};
private static void GroupDemo()
{
//按照名字的前两个字进行分组
var query = from custo in clist
group custo by custo.Name.Substring(0, 2);
foreach (IGrouping<string, CustomerInfo> group in query)
{
Console.WriteLine("分组键：{0}", group.Key);
foreach (var ci in group)
{
Console.WriteLine("姓名：{0}电话：{1}", ci.Name, ci.Tel);
}
Console.WriteLine("*********************");
}
//可以知道group子句返回的是一个IGrouping<TKey,TElement>泛型接口的对象集合
//TKey是键的对象类型，在用于group子句的时候，编译器会识别数据类型，用于存储分组的键值,也就是根据什么分的组
//TElement是指的对象类型用于分配储存结果，变量基于这个接口的类型就是遍历这个值，也就是分组的对象
}
//----------------into子句---------------//
private static void IntoDemo()
{
//into提供了一个临时标识符，它储存了into子句前面的查询内容，使他后面的子句可以方便使用，再次查询投影
var query = from custo in clist
group custo by custo.Name.Substring(0, 2) into gpcustomer
orderby gpcustomer.Key descending //排序，
select gpcustomer;
Console.WriteLine("into用于group子句");
foreach (var group in query)
{
Console.WriteLine("分组见：{0}", group.Key);
foreach (var ci in group)
{
Console.WriteLine("姓名：{0}电话：{1}", ci.Name, ci.Tel);
}
Console.WriteLine("***********************");
}
var query2 = from custo in clist
select new { NewName = custo.Name, NewAge = custo.Age } into newCustomer
orderby newCustomer.NewAge
select newCustomer;
Console.WriteLine("into用于select子句");
foreach (var ci in query2)
{
Console.WriteLine("{0}年龄：{1}", ci.NewName, ci.NewAge);
}
}
//---------------排序子句--------------------//
//LINQ可以按元素的一个或者多个属性对元素进行排序，表达式的排序方式分为OrderBy、OrderByDescending、ThenBy、ThenByDescending
//加了Descending的就是降序，没有加的就是升序
private static void ThenByDemo()
{
List<CustomerInfo> clist = new List<CustomerInfo>
{
new CustomerInfo{ Name="欧阳晓晓 ", Age=35, Tel ="1330708****"},
new CustomerInfo{ Name="上官飘飘 ", Age=17, Tel ="1592842****"},
new CustomerInfo{ Name="郭靖 ", Age=17, Tel ="1330708****"},
new CustomerInfo{ Name="黄蓉 ", Age=17, Tel ="1300524****"}
};
//按照年龄升序，再按照名字的字数次要排序
var query = from customer in clist
orderby customer.Age, customer.Name.Length
select customer;
Console.WriteLine("按年龄排列，按名字字数进行次要排序");
foreach (var ci in query)
{
Console.WriteLine("姓名:{0} 年龄：{1} 电话：{2}",ci.Name, ci.Age, ci.Tel);
}
//按年龄降序，再按名字的字数降序次要排列
var query2 = from customer in clist
orderby customer.Age descending , customer.Name.Length descending
select customer;
Console.WriteLine("\n按年龄排列，按名字字数进行降序次要排列");
foreach (var ci in query2)
{
Console.WriteLine("姓名：{0} 年龄：{1} 电话：{2}", ci.Name, ci.Age, ci.Tel);
}
}
//--------------let子句---------------------//
private static void LetDemo()
{
var query = from custo in clist
let g = custo.Name.Substring(0, 1)//let建立一个范围变量，在where中使用
where g == "欧" || g == "上"//也可以不写，写成customer.Name.Substring(0, 1) == "郭" || customer.Name.Substring(0, 1) == "黄"
select custo;
foreach (var ci in query)
{
Console.WriteLine("姓名：{0} 年龄：{1} 电话：{2}", ci.Name, ci.Age, ci.Tel);
}
}
//-------------join子句-------------------//
private static void JoinDemo()
{
//如果两个数据源中的属性可以进行相等比较，那么两个句子可以用join进行关联，比较的符号为equal，而不是==
List<CustomerInfo> clist = new List<CustomerInfo>
{
new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708****"},
new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842****"},
new CustomerInfo{ Name="郭靖", Age=17, Tel ="1330708****"},
new CustomerInfo{ Name="黄蓉", Age=17, Tel ="1300524****"}
};
List<CustomerTitle> titleList = new List<CustomerTitle>
{
new CustomerTitle{ Name="欧阳晓晓", Title="歌手"},
new CustomerTitle{ Name="郭靖", Title="大侠"},
new CustomerTitle{ Name="郭靖", Title="洪七公徒弟"},
new CustomerTitle{ Name="黄蓉", Title="才女"},
new CustomerTitle{ Name="黄蓉", Title="丐帮帮主"}
};
//根据姓名进行内部联结
var query = from customer in clist
join title in titleList
on customer.Name equals title.Name
select new { Name = customer.Name, Age = customer.Age, Title = title.Title };
foreach (var ci in query)
{
Console.WriteLine("姓名：{0} 年龄：{1}{2}", ci.Name, ci.Age, ci.Title);
}
//根据姓名进行分组联结
Console.WriteLine("\n根据姓名进行分组联结");
var query2 = from customer in clist
join title in titleList
on customer.Name equals title.Name into tgroup
select new { Name = customer.Name, Titles = tgroup };
foreach (var g in query2)
{
Console.WriteLine(g.Name);
foreach (var g2 in g.Titles)
{
Console.WriteLine(" {0}", g2.Title);
}
}
//根据姓名进行 左外部联结
Console.WriteLine("\n左外部联结");
var query3 = from customer in clist
join title in titleList
on customer.Name equals title.Name into tgroup
from subTitle in tgroup.DefaultIfEmpty()
select new { Name = customer.Name, Title = (subTitle == null ? "空缺" : subTitle.Title) };
foreach (var ci in query3)
{
Console.WriteLine("姓名：{0} ", ci.Name, ci.Title);
}
}
public class CustomerTitle
{
public string Name { get; set; }
public string Title { get; set; }
}
}
}
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