上篇C#语法之Linq查询基础一基本把Linq介绍了一下，这篇主要是列举下它的几个常见用法。
在用之前先准备些数据，新建了两个类Student、Score，并通过静态方法提供数据。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks; namespace LinqDemo
{
public class Student
{
public string StuId { get; set; } public string Name { get; set; } public int Age { get; set; } public Student(string stuId, string name,int age)
{
StuId = stuId;
Name = name;
Age = age;
} public static List<Student> GetAllStudents()
{
List<Student> stus = new List<Student>() {
new Student("","xiaoming1",),
new Student("","xiaoming2",),
new Student("","xiaoming3",),
new Student("","xiaoming4",),
new Student("","xiaoming5",),
new Student("","xiaoming1",)
};
return stus;
}
}
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks; namespace LinqDemo
{
public class Score
{
public string StuId { get; set; } public int Math { get; set; } public int English { get; set; } public int Chinese { get; set; } public Score(string stuId, int math, int english, int chinese)
{
StuId = stuId;
Math = math;
English = english;
Chinese = chinese;
} public static List<Score> GetAllScores()
{
List<Score> scores = new List<Score>()
{
new Score("",,,),
new Score("",,,),
new Score("",,,),
new Score("",,,),
new Score("",,,)
};
return scores;
} }
}
一、筛选
where 是筛选lamdba表达式的，OfType<TResult>是筛选TResult类型的
int[] a = new int[] { , , , , , , , , , };
var stus = Student.GetAllStudents().Where(p => p.StuId.Equals(""));
foreach (Student stu in stus)
{
Console.WriteLine("StuId:{0} Name:{1}", stu.StuId, stu.Name);
}
var result = a.Where((r, index) => r % == && index % == );
foreach (var s in result)
{
Console.WriteLine("{0}", s);
}
Console.WriteLine("--------------------------");
object[] data = { "one", , , "four", };
//OfType 可隐式转换 sting可隐式转换为int int不可隐式转换为string 所以不能直接写OfType<string>()
result = data.OfType<int>();
foreach (var s in result)
{
Console.WriteLine("{0}", s);
}
二、改变元素顺序
Orderby thenby来进行排序，thenby可以使用多次来多条件排序
//单个排序
int[] a = new int[] { , , , , , , , , , };
var result = a.OrderByDescending(p => p);
foreach (var s in result)
{
Console.WriteLine("{0}", s);
}
//多条件排序
var result = Student.GetAllStudents().OrderByDescending(p => p.StuId).ThenBy(p => p.Name); foreach (var s in result)
{
Console.WriteLine("StuId:{0} Name:{1}", s.StuId, s.Name);
}
翻转
对于上面的查询后面加一个Reverse()方法则会将结果翻转。
var result = Student.GetAllStudents().OrderByDescending(p => p.StuId).ThenBy(p => p.Name).Reverse();
foreach (var s in result)
{
Console.WriteLine("StuId:{0} Name:{1}", s.StuId, s.Name);
}
三、分组
分组常用作统计或查找重复。下面的例子就是查找姓名和年龄都相同的学生。
var result = from stu in Student.GetAllStudents()
group stu by new { stu.Name, stu.Age } into g
where g.Count() >
select new
{
g.Key.Name,
g.Key.Age
}; foreach (var s in result)
{
Console.WriteLine("Name:{0} Age:{1}", s.Name, s.Age);
}
四、连接
左连接
var result = from stu in Student.GetAllStudents()
join s in Score.GetAllScores() on stu.StuId equals s.StuId into joinStuScore
from p in joinStuScore.DefaultIfEmpty()
select new
{
StuId = stu.StuId,
Name = stu.Name,
Math = p == null ? : p.Math,
English = p == null ? : p.English,
Chinese = p == null ? : p.Chinese };
foreach (var s in result)
{
Console.WriteLine("StuId:{0} Name:{1} Math:{2} English:{3} Chinese:{4}", s.StuId, s.Name, s.Math, s.English, s.Chinese);
}
内连接
var result = from stu in Student.GetAllStudents()
join s in Score.GetAllScores() on stu.StuId equals s.StuId select new
{
StuId = stu.StuId,
Name = stu.Name,
Math = s.Math,
English = s.English,
Chinese = s.Chinese };
foreach (var s in result)
{
Console.WriteLine("StuId:{0} Name:{1} Math:{2} English:{3} Chinese:{4}", s.StuId, s.Name, s.Math, s.English, s.Chinese);
}
五、集合操作
下面是两个集合的交集、差集和并集，至于distinct这个之前有专门讲解。
int[] a = {,,, };
int[] b = { ,,,};
var result = a.Intersect(b);
foreach (var r in result)
{
Console.WriteLine(r);
}
Console.WriteLine("---------------");
result=a.Except(b);
foreach (var r in result)
{
Console.WriteLine(r);
}
Console.WriteLine("---------------");
result = a.Union(b);
foreach (var r in result)
{
Console.WriteLine(r);
}
六、分区
Take()和Skip()常用来做分页操作。下面的demo演示分页。
int pageSize = ;
int pageCount = (int)Math.Ceiling(Student.GetAllStudents().Count()/(double)pageSize);
for (int pageIndex = ; pageIndex < pageCount; pageIndex++)
{
Console.WriteLine("page {0}",pageIndex);
var result = (from s in Student.GetAllStudents().OrderBy(p => p.Age) select s).Skip(pageIndex * pageSize).Take(pageSize); foreach (var s in result)
{
Console.WriteLine("StuId:{0} Name:{1} Age:{2}", s.StuId, s.Name,s.Age);
}
}
七、限定符操作符
Any、All、Contains都是限定符操作符。Any是否有一个满足条件。ALL是所有元素都满足条件。Contains检查某个元素是否在集合中。都是返回布尔值。
bool anyFlag = Student.GetAllStudents().Any(p=>p.Name.Equals("xiaoming1") &&p.Age==);
bool allFlag = Student.GetAllStudents().Any(p =>p.Age == );
Student s = new Student("","xiaoming",);
bool containsFlag = Student.GetAllStudents().Contains(s);
Console.WriteLine("Any:{0} All:{1} Contains:{2}",anyFlag,allFlag,containsFlag);
八、聚合函数
Linq还提供了Count()、Sum()、Min()、Max()、Average()、Aggregate()一系列聚合函数。