C#语法之Linq查询基础二

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("001","xiaoming1",25),
                new Student("002","xiaoming2",24),
                new Student("003","xiaoming3",23),
                new Student("004","xiaoming4",26),
                new Student("005","xiaoming5",27),
                new Student("006","xiaoming1",25)
            };
            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("001",85,90,85),
                new Score("002",85,90,85),
                new Score("003",60,70,65),
                new Score("004",59,99,75),
                new Score("005",66,90,65)
            };
            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[] { 1, 3, 4, 6, 8, 7, 9, 1, 3, 2 };
            var stus = Student.GetAllStudents().Where(p => p.StuId.Equals("001"));
            foreach (Student stu in stus)
            {
                Console.WriteLine("StuId:{0} Name:{1}", stu.StuId, stu.Name);
            }

            var result = a.Where((r, index) => r % 2 == 0 && index % 2 == 1);
            foreach (var s in result)
            {
                Console.WriteLine("{0}", s);
            }
            Console.WriteLine("--------------------------");
            object[] data = { "one", 2, 3, "four", 5 };
            //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[] { 1, 3, 4, 6, 8, 7, 9, 1, 3, 2 };
            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() > 1
                         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 ? 0 : p.Math,
                             English = p == null ? 0 : p.English,
                             Chinese = p == null ? 0 : 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 = {1,3,5,2 };
            int[] b = { 1,2,6,7};
            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 = 5;
            int pageCount = (int)Math.Ceiling(Student.GetAllStudents().Count()/(double)pageSize);
            for (int pageIndex = 0; 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==25);
            bool allFlag = Student.GetAllStudents().Any(p =>p.Age == 25);
            Student s = new Student("001","xiaoming",25);
            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。