c# Linq查询
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：
- 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();
}