【C#】LINQ学习笔记

　　LINQ（Language Integrated Query）全称基于关系数据的 .NET 语言集成查询，用于以对象形式管理关系数据，并提供了丰富的查询功能，它和Linq to xml、Linq to objects、Linq to dataset、Linq to entities等组成了强大的LINQ。

      LINQ是C# 3.0引入的新特性，C#3.0也引入了其他一些新特性，以更好的使用LINQ，LinQ说白了就是使我们可以更方便更快捷的操作集合

1、基础特性

　　1、自动属性

    public class Student
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

　　2、对象初始化器

    Student toroto = new Student {Id = 1, , Name = "toroto"};
    Student bomo = new Student {Id = 2, , Name = "bomo"};

　　3、集合初始化器

        List<Student> students = new List<Student>
            {
                new Student {Id = 1, Name = "toroto"},
                new Student {Id = 2, Name = "bomo"}
            };

        Dictionary<string, Student> dictionary = new Dictionary<string, Student>
            {
                {"toroto", new Student {Id = 1, Name = "toroto"}},
                {"bomo", new Student {Id = 2, Name = "bomo"}}
            };

　　4、隐式类型

        //编译器自动根据上下文判断类型，变量不能初始为null
        var i = 10;
        var f = 10.5f;
        var b = true;
        var student = new Student();
        var list = new List<Student>();

　　5、匿名类型对象

        //创建匿名类型的对象，该对象有两个属性（Id，Name）(注意：不是Student类型)
        var stu = new {Id = 1, Name = "toroto"};

　　6、Lambda表达式（匿名函数）

　　　　(参数列表) => {函数体} 　　　　如：　(a, b) => { return a + b; };

　　7、扩展方法（需要.Net 3.5）

        ////扩展方法类:必须为非嵌套,非泛型的静态类
        public static class DatetimeEx
        {
            //通过this声明扩展的类，这里给DateTime类扩展一个Show方法，只有一个参数
            public static void Show(this DateTime date, string msg)
            {
                Console.WriteLine("扩展方法调用");
                Console.WriteLine(msg);
            }
        }

　　　　这样DateTime类就多了一个Show()方法了

2、LinQ To Object

　　接下来才是重点，LinQ的使用很简单，很简洁明了，查询到的结果使用 foreach 遍历

　　首先定义几个集合，用于下面查询

        public class Student
        {
            public int Id { get; set; }
            public int Age { get; set; }
            public int ClassNum { get; set; }
            public string Name { get; set; }
        }

        var students = new List<Student>
            {
                new Student {Id = 2, Age = 22, ClassNum = 2, Name = "bomo"},
                new Student {Id = 1, Age = 21, ClassNum = 1, Name = "toroto"},
                new Student {Id = 3, Age = 19, ClassNum = 2, Name = "tobi"},
                new Student {Id = 4, Age = 20, ClassNum = 1, Name = "cloud"}
            };

　　1、简单查询　　　　

        //从studnets中选择年龄>20的学生,并按降序排列
        var res = from student in students          //选择集合
                  where student.Age > 20            //条件
                  orderby student.Age descending    //升序：ascending 降序:descending
                  select student;                   //结果集合
        foreach (var student in res)
            Console.WriteLine("{0}:{1}", student.Name, student.Age);

 

　　2、分组查询（使用了匿名类型对象）

        //学生按ClassNum分组
        var groups = from student in students
                     group student by student.ClassNum
                     into Class
                     select new
                         {
                             Students = Class,
                             ClassNum = Class.Key
                         };
        //第一次遍历到分组,第二次遍历才是学生
        foreach (var group in groups)
        {
            Console.WriteLine("Class {0}:", group.ClassNum);
            foreach (var student in group.Students)
            {
                Console.WriteLine(student.Name);
            }
        }

　　3、多条件排序

        //先按班级排序（升序），班级相同则按年龄排序（降序）
        var res = from student in students
                  orderby student.ClassNum ascending, student.Age descending 
                  select student;

        foreach (var student in res)
            Console.WriteLine("Name:{0}, Age:{1}, ClassNum:{2}", student.Name, student.Age, student.ClassNum);

　　4、多集合查询

        int[] numbersA = {0, 2, 4, 5, 6, 8, 9};
        int[] numbersB = {1, 3, 5, 7, 8};
        //多个集合的比较，相当于多重遍历
        var pairs =
            from a in numbersA
            from b in numbersB
            let diff = Math.Abs(a-b)            //中间值/额外值 或 操作
            where a < b
            select new {a, b, diff = diff};

        foreach (var pair in pairs)
            Console.WriteLine("{0}>{1}：{2}", pair.a, pair.b, pair.diff);

　　5、连接查询

　　　　定义两个类

        public class Student
        {
            public int Id { get; set; }
            public int Age { get; set; }
            public int ClassNum { get; set; }
            public string Name { get; set; }
        }

        public class Score
        {
            public int Id { get; set; }
            public int ChineseScore { get; set; }
            public int MathScore { get; set; }
            public int EnglishScore { get; set; }
        }

　　　　使用 join 查询

        var students = new List<Student>
            {
                new Student {Id = 1, Age = 21, ClassNum = 1, Name = "toroto"},
                new Student {Id = 2, Age = 22, ClassNum = 2, Name = "bomo"},
                new Student {Id = 3, Age = 19, ClassNum = 2, Name = "tobi"},
                new Student {Id = 4, Age = 20, ClassNum = 1, Name = "cloud"}
            };
        var scores = new List<Score>
            {
                new Score { Id = 1, ChineseScore = 77, MathScore = 80, EnglishScore = 85},
                new Score { Id = 2, ChineseScore = 40, MathScore = 90, EnglishScore = 75},
                new Score { Id = 3, ChineseScore = 68, MathScore = 30, EnglishScore = 80},
                new Score { Id = 4, ChineseScore = 85, MathScore = 100, EnglishScore = 88},
            };

        var res = from student in students
                  join score in scores on student.Id equals score.Id
                  select new {Student = student, Score = score};

        foreach (var student in res)
        {
            Console.WriteLine("{0}:ChineseScore:{1}, MathScore:{2}, EnglishScore:{3}", student.Student.Name,
                              student.Score.ChineseScore, student.Score.MathScore, student.Score.EnglishScore);
        }

　　6、分组连接查询

        var students = new List<Student>
            {
                new Student {Id = 1, Age = 21, ClassNum = 1, Name = "toroto"},
                new Student {Id = 2, Age = 22, ClassNum = 2, Name = "bomo"},
                new Student {Id = 3, Age = 19, ClassNum = 2, Name = "tobi"},
                new Student {Id = 4, Age = 20, ClassNum = 1, Name = "cloud"}
            };
        var scores = new List<Score>
            {
                new Score { Id = 1, ChineseScore = 77, MathScore = 80, EnglishScore = 85},
                new Score { Id = 1, ChineseScore = 87, MathScore = 90, EnglishScore = 95},
                new Score { Id = 2, ChineseScore = 40, MathScore = 90, EnglishScore = 75},
                new Score { Id = 3, ChineseScore = 68, MathScore = 30, EnglishScore = 80},
                new Score { Id = 4, ChineseScore = 85, MathScore = 100, EnglishScore = 88},
            };

        var res = from student in students
                  join score in scores on student.Id equals score.Id into Sco
                  select new {Student = student, Score = Sco};

        foreach (var student in res)
        {
            Console.WriteLine("{0}:", student.Student.Name);
            foreach (var score in student.Score)
            {
                Console.WriteLine("ChineseScore:{0}, MathScore:{1}, EnglishScore:{2}", score.ChineseScore,
                                  score.MathScore, score.EnglishScore);
            }
        }

　　由于数据库不是很熟，所以这里只记录LinQ To Object，应该很多操作都是相同的，下次在写LinQ To SQL

LinQ还有使用函数查询的操作，内容比较多，明天再写，今天就写到这里