C#知识整理-LINQ

合集 - C#知识整理(10)
1.C#知识整理-运算符2.C#知识整理-语句3.C#知识整理-类型4.C#知识整理-类（Class）5.C#知识整理-泛型6.C#知识整理-文件处理(基础)7.C#知识整理-多线程8.C#知识整理-异步编程9.C#知识整理-垃圾回收机制(GC)
10.C#知识整理-LINQ
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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。 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。
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。
LINQ 查询操作都由以下三个不同的操作组成：
  1. 获取数据源。
  2. 创建查询。
  3. 执行查询。
下面用一个演示表示查询操作的三个部分
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// 1. Data source.
List<int> numbers = new List<int>() { 0, 1, 2, 3, 4, 5, 6 };
//int[] numbers = [0, 1, 2, 3, 4, 5, 6];
// 2. Query creation.
// numQuery is an IEnumerable<int>
var numQuery = from num in numbers
               where (num % 2) == 0
               select num;
// 3. Query execution.
foreach (int num in numQuery)
{
    Console.Write("{0,1} ", num);
}
// output 0 2 4 6

为什么要使用LINQ，用上述的案例，如果不用LINQ如何来写

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List<int> numbers = new List<int>() { 0, 1, 2, 3, 4, 5, 6 };
var numResults = new List<int>();
foreach (var num in numbers) {
    if (num % 2 == 0) {
        numResults.Add(num);
    }
}
foreach (int num in numResults)
{
    Console.Write("{0,1} ", num);
}
// output 0 2 4 6
如果不使用LINQ，筛选数据只能通过定义一个空的结果集，然后通过for循环和if语句把符合条件的结果插入到结果集里，
然后打印出结果
除此之外，我们还可以使用Enumerable自带的where子句达到相同的效果，可以使用lambda表达式
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List<int> numbers = new List<int>() { 0, 1, 2, 3, 4, 5, 6 };
var numQuery1 = numbers.Where(p => p % 2 == 0);
foreach (int num in numQuery1)
{
    Console.Write("{0,1} ", num);
}
// output 0 2 4 6

下面用一个例子展示一下常用的查询

首先定义一些数据集，这些数据可以是从数据库或调用API得到的，我们就定义一下静态数据
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internal class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public int ClassId { get; set; }
 
    public override string ToString()
    {
        return $"Id:{Id},Name:{Name},Age:{Age},ClassId:{ClassId}";
    }
}
 
internal class Lesson
{
    public int Id { get; set; }
    public string Name { get; set; }
}
 
 
internal class StudentLesson
{
    public int StudentId { get; set; }
    public int LessonId { get; set; }
    public decimal Score { get; set; }
}
 
internal static class StudentData {
    public static IEnumerable<Student> students;
    public static IEnumerable<Lesson> lessons;
    public static IEnumerable<StudentLesson> studentLessons;
 
    public static void init() {
        List<Lesson> lstLessons = new List<Lesson>
        {
            new Lesson() {
                Id=1,
                Name="语文"
            },
            new Lesson() {
                Id=2,
                Name="数学"
            },
            new Lesson() {
                Id=3,
                Name="英语"
            },
            new Lesson() {
                Id=4,
                Name="物理"
            },
        };
        List<Student> lstStudents = new List<Student>() {
            new Student(){
                Id = 1,
                Age = 12,
                ClassId = 1,
                Name = "张一"
            },
            new Student(){
                Id = 2,
                Age = 12,
                ClassId = 1,
                Name = "张二"
            },
            new Student(){
                Id = 3,
                Age = 12,
                ClassId = 1,
                Name = "张三"
            },
            new Student(){
                Id = 4,
                Age = 12,
                ClassId = 1,
                Name = "张四"
            },
            new Student(){
                Id = 5,
                Age = 14,
                ClassId = 2,
                Name = "王一"
            },
            new Student(){
                Id = 6,
                Age = 14,
                ClassId = 2,
                Name = "王二"
            },
            new Student(){
                Id = 7,
                Age = 14,
                ClassId = 2,
                Name = "王三"
            },
            new Student(){
                Id = 8,
                Age = 14,
                ClassId = 2,
                Name = "王四"
            },
            new Student(){
                Id = 9,
                Age = 13,
                ClassId = 3,
                Name = "李一"
            },
            new Student(){
                Id = 10,
                Age = 13,
                ClassId = 3,
                Name = "李二"
            },
            new Student(){
                Id = 11,
                Age = 13,
                ClassId = 3,
                Name = "李三"
            },
            new Student(){
                Id = 12,
                Age = 13,
                ClassId = 3,
                Name = "李四"
            },
        };
        List<StudentLesson> lstStudentLessons = new List<StudentLesson>();
        foreach (var lesson in lstLessons) {
            foreach (var student in lstStudents) {
                StudentLesson studentLesson = new StudentLesson()
                {
                    LessonId = lesson.Id,
                    StudentId = student.Id,
                };
                var score = 0.0m;
                switch (student.Id)
                {
                    case 1:
                        score = 80.3m; break;
                    case 2:
                        score = 90.5m; break;
                    case 3:
                        score = 70.2m; break;
                    case 4:
                        score = 83.7m; break;
                }
                studentLesson.Score = score;
 
                lstStudentLessons.Add(studentLesson);
            }
        }
 
        students = lstStudents;
        lessons = lstLessons;
        studentLessons = lstStudentLessons;
    }
}

现在我们就根据定义好的数据进行查询

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StudentData.init();
var students = StudentData.students;
var lessons = StudentData.lessons;
var studentLessons = StudentData.studentLessons;
 
//查询
var zhangStudents = students.Where(p => p.Name.Contains("张"));
foreach (var student in zhangStudents)
{
    Console.WriteLine(student.Name);
}
/*output:
    张一
    张二
    张三
    张四
 */
var zhangStudents1 = from student in students
                     where student.Name.Contains("张")
                     select student;
foreach (var student in zhangStudents1)
{
    Console.WriteLine(student.Name);
}
/*output:
    张一
    张二
    张三
    张四
 */
  
// group by
var groupResult = students.GroupBy(p => p.ClassId).ToList();
foreach (var group in groupResult)
{
    Console.WriteLine($"ClassId: {group.Key}");
    foreach (var student in group)
    {
        Console.WriteLine(student);
    }
}
/*
    ClassId: 1
    Id:1,Name:张一,Age:12,ClassId:1
    Id:2,Name:张二,Age:12,ClassId:1
    Id:3,Name:张三,Age:12,ClassId:1
    Id:4,Name:张四,Age:12,ClassId:1
    ClassId: 2
    Id:5,Name:王一,Age:14,ClassId:2
    Id:6,Name:王二,Age:14,ClassId:2
    Id:7,Name:王三,Age:14,ClassId:2
    Id:8,Name:王四,Age:14,ClassId:2
    ClassId: 3
    Id:9,Name:李一,Age:13,ClassId:3
    Id:10,Name:李二,Age:13,ClassId:3
    Id:11,Name:李三,Age:13,ClassId:3
    Id:12,Name:李四,Age:13,ClassId:3
 */
  
 // group by
var groupResult = students.GroupBy(p => p.ClassId).ToList();
                   
foreach (var group in groupResult)
{
    Console.WriteLine($"ClassId: {group.Key}");
    foreach (var student in group)
    {
        Console.WriteLine(student);
    }
}
/*
    ClassId: 1
    Id:1,Name:张一,Age:12,ClassId:1
    Id:2,Name:张二,Age:12,ClassId:1
    Id:3,Name:张三,Age:12,ClassId:1
    Id:4,Name:张四,Age:12,ClassId:1
    ClassId: 2
    Id:5,Name:王一,Age:14,ClassId:2
    Id:6,Name:王二,Age:14,ClassId:2
    Id:7,Name:王三,Age:14,ClassId:2
    Id:8,Name:王四,Age:14,ClassId:2
    ClassId: 3
    Id:9,Name:李一,Age:13,ClassId:3
    Id:10,Name:李二,Age:13,ClassId:3
    Id:11,Name:李三,Age:13,ClassId:3
    Id:12,Name:李四,Age:13,ClassId:3
 */
 
var groupResult1 = from student in students
                    group student by student.ClassId into groupString
                    orderby groupString.Key descending
                    select groupString;
foreach (var group in groupResult1)
{
    Console.WriteLine($"ClassId: {group.Key}");
    foreach (var student in group)
    {
        Console.WriteLine(student);
    }
}
/* output
    ClassId: 3
    Id: 9,Name: 李一,Age: 13,ClassId: 3
    Id: 10,Name: 李二,Age: 13,ClassId: 3
    Id: 11,Name: 李三,Age: 13,ClassId: 3
    Id: 12,Name: 李四,Age: 13,ClassId: 3
    ClassId: 2
    Id: 5,Name: 王一,Age: 14,ClassId: 2
    Id: 6,Name: 王二,Age: 14,ClassId: 2
    Id: 7,Name: 王三,Age: 14,ClassId: 2
    Id: 8,Name: 王四,Age: 14,ClassId: 2
    ClassId: 1
    Id: 1,Name: 张一,Age: 12,ClassId: 1
    Id: 2,Name: 张二,Age: 12,ClassId: 1
    Id: 3,Name: 张三,Age: 12,ClassId: 1
    Id: 4,Name: 张四,Age: 12,ClassId: 1
 */
  
  
// join
var joinresult = from student in students
             join studentLesson in studentLessons
             on student.Id equals studentLesson.StudentId
             join lesson in lessons
             on studentLesson.LessonId equals lesson.Id
             where lesson.Name.Equals("语文")
             select new
             {
                 student.Id,
                 student.Name,
                 student.ClassId,
                 student.Age,
                 LessonName = lesson.Name,
                 Score = studentLesson.Score
             };
foreach (var result in joinresult) {
    Console.WriteLine($"id:{result.Id},name:{result.Name},class id:{result.ClassId},age:{result.Age},lesson name:{result.LessonName},score:{result.Score}");
}
/*
* output
id:1,name:张一,class id:1,age:12,lesson name:语文,score:80.3
id:2,name:张二,class id:1,age:12,lesson name:语文,score:90.5
id:3,name:张三,class id:1,age:12,lesson name:语文,score:70.2
id:4,name:张四,class id:1,age:12,lesson name:语文,score:83.7
id:5,name:王一,class id:2,age:14,lesson name:语文,score:0.0
id:6,name:王二,class id:2,age:14,lesson name:语文,score:0.0
id:7,name:王三,class id:2,age:14,lesson name:语文,score:0.0
id:8,name:王四,class id:2,age:14,lesson name:语文,score:0.0
id:9,name:李一,class id:3,age:13,lesson name:语文,score:0.0
id:10,name:李二,class id:3,age:13,lesson name:语文,score:0.0
id:11,name:李三,class id:3,age:13,lesson name:语文,score:0.0
id:12,name:李四,class id:3,age:13,lesson name:语文,score:0.0
*/

　　

posted @   Terry841119  阅读(12)  评论(0)  编辑  收藏  举报
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