Linq 是C# 3.0 新增的好东西，用来处理对象集合，可用来查询数据库，XML文档
主要分为 Linq To Objects （用来处理内存对象集合），也是本节的重点
Linq TO SQL
Linq TO XML
Lnq To Providers
1. 基本语法
var n=
from 数据源
where ..
select n;
最后 foreach( var a in n) 循环结果集，例子将展示最基本的用法.
例子namespace Linq_1
{
class Program
{
static void Main(string[] args)
{
string[] strNames = { "小明","小李","小王","大胡","大大"};
var queryResults =
from n in strNames
where n.StartsWith("小")
select n;
Console.WriteLine("名子以小开头的有: ");
foreach (var item in queryResults)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
}
}
声明字符串数组 strNames； 然后linq表达式 from 指定数据源,where 指明条件,select n 原则结果集
最后通过 foreach 获取结果集，延迟执行！将名字中以“小”开头的名字打印出来。
2. 用Linq方法语法和λ表达式（n=>xx）
1) λ表达式
n=>n<100 就是匿名函数，n为参数，n<100是函数或者方法体。若n<100则返回Ture
2)Linq方法语法
同样例子查找以“小”开头的人名
var queryResult=strNames.where(n=>n.startwith("小"))；
Where方法static void Main(string[] args)
{
string[] strNames = { "小明", "小李", "小王", "大胡", "大大" };
var queryResults = strNames.Where(n => n.StartsWith("小"));
Console.WriteLine("名子以小开头的有: ");
foreach (var item in queryResults)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
方法：OrderBy
OrderBy方法var queryResults2 = strNames.OrderBy(n => n).Where(n => n.StartsWith("小"));
foreach (var item in queryResults2)
{
Console.WriteLine(item);
}
3. 合计运算符
Count(),Min(),Max(),Average(),Sum()
合计运算符 class Program
{
static void Main(string[] args)
{
int[] numbers = generateLotsOfNumbers(12345678);
//小于1000的数字
var queryResults =
from n in numbers
where n < 1000
select n;
Console.WriteLine("小于1000的数有：");
foreach (var str in queryResults)
{
Console.WriteLine(str);
}
//合计运算符操作
var queryResults2 =
from n in numbers
where n > 1000
select n;
Console.WriteLine("Count of numbers >1000 ");
Console.WriteLine(queryResults2.Count());
Console.WriteLine("min of numbers >1000");
Console.WriteLine(queryResults2.Min());
Console.WriteLine("max of numbers >1000");
Console.WriteLine(queryResults2.Max());
Console.WriteLine("average of numbers >1000");
Console.WriteLine(queryResults2.Average());
Console.WriteLine("sum of numbers >1000");
Console.WriteLine(queryResults2.Sum(n=>(long)n));
Console.ReadKey();
}
/// <summary>
/// 生成一个随机数列表
/// </summary>
/// <param name="count"></param>
/// <returns></returns>
private static int[] generateLotsOfNumbers(int count)
{
Random generator = new Random(0);
int[] result = new int[count];
for (int i = 0; i < count; i++)
{
result[i] = generator.Next();
}
return result;
}
}
查询复杂对象using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Linq_查询复杂对象
{
class Customer
{
public string ID { get; set; }
public string City { get; set; }
public string Country { get; set; }
public string Region {get;set; }
public decimal Sales{get;set;}
public override string ToString()
{
return "ID: " + ID + " City" + City + " Country" + Country + " Region" + Region + " Sales" + Sales;
}
}
class Program
{
static void Main(string[] args)
{
//查询asia亚洲
List<Customer> customers = new List<Customer>
{
new Customer{ID="A",City="New York",Country="USA",Region="North America",Sales=9999},
new Customer{ID="B",City="Mumbai",Country="India",Region="Asia",Sales=8888},
new Customer{ID="C",City="Karachi",Country="Pakistan",Region="Asia",Sales=7777},
};
var queryResults =
from c in customers
where c.Region == "Asia"
select c;
Console.WriteLine("Customers in Asia:");
foreach (var a in queryResults)
{
Console.WriteLine(a);
}
Console.ReadKey();
}
}
}
4. 投影：查询中创建对象
select n+1
select n.ToUpper()
错误：select n.name,n.city 只允许select one.所以当要查询多个数据时候应该创建对象
select new {n.name,n.city}
投射方法语法-在查询中创建对象using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
//投射.在查询中创建新对象
namespace Linq_投射.在查询中创建新对象
{
class Customer
{
public string ID { get; set; }
public string City { get; set; }
public string Country { get; set; }
public string Region { get; set; }
public decimal Sales { get; set; }
public override string ToString()
{
return "ID: " + ID + " City" + City + " Country" + Country + " Region" + Region + " Sales" + Sales;
}
}
class Program
{
static void Main(string[] args)
{
//查询asia亚洲
List<Customer> customers = new List<Customer>
{
new Customer{ID="A",City="New York",Country="USA",Region="North America",Sales=9999},
new Customer{ID="B",City="Mumbai",Country="India",Region="Asia",Sales=8888},
new Customer{ID="C",City="Karachi",Country="Pakistan",Region="Asia",Sales=7777},
};
var queryResults =
from c in customers
where c.Region == "Asia"
select new { c.City, c.Country, c.Sales }; //投射.在查询中创建新对象
var queryResults2 = customers.Where(c => c.Region == "Asia").Select(c => new { c.City, c.Country, c.Sales });
Console.WriteLine("Customers in Asia:");
foreach (var a in queryResults)
{
Console.WriteLine(a);
}
Console.WriteLine("Customers in Asia:");
foreach (var a in queryResults2)
{
Console.WriteLine(a);
}
Console.ReadKey();
}
}
}
5. 单值选项 select(n=>new { xx}).Distinct();
6. Any()和All() 查找数据源是否有，有就Ture，否则false
例：查找 是否有 集合项C的country 是否为 “USA”
bool anyUsa=Customers.Any(n=>n.country=="USA");
7. 多级排序 OrderBy(x).ThenBy(x)
多级排序using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
//orderby(c=>xx).ThenBy(c=>c.xx)；
// Take()类似SQL 的top,skip()跳过前几个结果;
//First,firstorDefault()
namespace Linq__多级排序
{
class Customer
{
public string ID { get; set; }
public string City { get; set; }
public string Country { get; set; }
public string Region { get; set; }
public decimal Sales { get; set; }
public override string ToString()
{
return "ID: " + ID + " City" + City + " Country" + Country + " Region" + Region + " Sales" + Sales;
}
}
class Program
{
static void Main(string[] args)
{
//查询asia亚洲
List<Customer> customers = new List<Customer>
{
new Customer{ID="A",City="New York",Country="USA",Region="North America",Sales=9999},
new Customer{ID="B",City="Mumbai",Country="India",Region="Asia",Sales=8888},
new Customer{ID="C",City="Karachi",Country="Pakistan",Region="Asia",Sales=7777},
};
//普通排序
var queryResults =
from c in customers
orderby c.Country,c.ID,c.City
select new { c.City, c.Country, c.Sales };
Console.WriteLine("Customers in Asia:");
foreach (var a in queryResults)
{
Console.WriteLine(a);
}
//多级排序方法
var queryResults2 = customers.OrderBy(c => c.Country).ThenBy(c => c.ID).ThenBy(c => c.Sales).Select(c => new { c.City, c.Country, c.Sales });
foreach (var a in queryResults2)
{
Console.WriteLine(a);
}
//Take() 取前1个结果
Console.WriteLine("queryResults2 的第一个结果是:");
foreach (var a in queryResults2.Take(1))
{
Console.WriteLine(a);
}
//Skip(int) 排除前几个结果
Console.WriteLine("queryResults2 出去第一个结果是:");
foreach (var a in queryResults2.Skip(1))
{
Console.WriteLine(a);
}
//First()
Console.WriteLine("queryResults2 coutry是USA 的第一个结果是:");
Console.WriteLine(queryResults2.First(b=>b.Country=="USA"));
Console.WriteLine("queryResults2 coutry是china 的第一个结果是:");
Console.WriteLine(queryResults2.FirstOrDefault(b => b.Country == "china"));
Console.ReadKey();
}
}
}
8. 组合查询
from c in xx
group c by c.Region into cq
select new {
TotalPrice=cq.Sum(n=>c.sales),
Region=cq.key
}
9. Take() 类似 MSSQL 的Top（），返回前n个结果
Skip（） 跳过前n个结果集
Console.WriteLine("queryResults2 的第一个结果是:");
foreach (var a in queryResults2.Take(1))
{
Console.WriteLine(a);
}
