LINQ 之 使用 LINQ 进行数据转换

语言集成查询 (LINQ) 不仅可用于检索数据，而且还是一个功能强大的数据转换工具。通过使用 LINQ 查询，您可以将源序列用作输入，并采用多种方式修改它以创建新的输出序列。您可以通过排序和分组来修改该序列，而不必修改元素本身。但是，LINQ 查询的最强大的功能是能够创建新类型。这一功能在 select 子句中实现。 例如，可以执行下列任务：  

 3.1 将多个输入联接到一个输出序列

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

        public int Age { get; set; }

        public string City { get; set; }

        public List<int> Scores { get; set; }
    }

    class Teacher
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public int Age { get; set; }

        public string City { get; set; }

    }

学生和老师两个类

internal class Program
    {
        private static void Main(string[] args)
        {
            //创建第一个数据源
            var students = new List<Student>()
            {
                new Student()
                {
                    Age = 23,
                    City = "广州",
                    Name = "小C",
                    Scores = new List<int>(){85,88,83,97}
                },
                new Student()
                {
                    Age = 18,
                    City = "广西",
                    Name = "小明",
                    Scores = new List<int>(){86,78,85,90}
                },
                new Student()
                {
                    Age = 33,
                    City = "梦里",
                    Name = "小叁",
                    Scores = new List<int>(){86,68,73,97}
                }
            };

            //创建第二个数据源
            var teachers = new List<Teacher>()
            {
                new Teacher()
                {
                    Age = 35,
                    City = "梦里",
                    Name = "啵哆"
                },
                new Teacher()
                {
                    Age = 28,
                    City = "云南",
                    Name = "小红"
                },
                new Teacher()
                {
                    Age = 38,
                    City = "河南",
                    Name = "丽丽"
                }
            };

            //创建查询
            var peopleInDreams = (from student in students
                            where student.City == "梦里"
                            select student.Name)
                            .Concat(from teacher in teachers
                                    where teacher.City == "梦里"
                                    select teacher.Name);

            //执行查询
            foreach (var person in peopleInDreams)
            {
                Console.WriteLine(person);
            }

            Console.Read();
        }
    }

控制台输出代码。

3.2 选择各个源元素的子集

1. 若要只选择源元素的一个成员，请使用点运算。

var query = from cust in Customers
                     select cust.City;

2. 若要创建包含源元素的多个属性的元素，可以使用具有命名对象或匿名类型的对象初始值设定项。

     var query = from cust in Customer
                    select new {Name = cust.Name, City = cust.City};

 3.3 将内存中的对象转换为 XML

//创建数据源
            var students = new List<Student>()
            {
                new Student()
                {
                    Age = 18,
                    Name = "小A",
                    Scores = new List<int>() {88,85,74,66 }
                },
                new Student()
                {
                    Age = 35,
                    Name = "小B",
                    Scores = new List<int>() {88,85,74,66 }
                },
                new Student()
                {
                    Age = 28,
                    Name = "小啥",
                    Scores = new List<int>() {88,85,74,66 }
                }
            };

            //创建查询
            var studentsToXml = new XElement("Root",
                from student in students
                let x = $"{student.Scores[0]},{student.Scores[1]},{student.Scores[2]},{student.Scores[3]}"
                select new XElement("student",
                new XElement("Name", student.Name),
                new XElement("Age", student.Age),
                new XElement("Scores", x))
            );

            //执行查询
            Console.WriteLine(studentsToXml);

 3.4 对源元素执行操作

　　输出序列可能不包含源序列的任何元素或元素属性。输出可能是通过将源元素用作输入参数计算出的值的序列。

            //数据源
            double[] radii = {1, 2, 3};
            //创建查询
            var query = from radius in radii
                select $"{radius * radius * 3.14}";
            //执行查询
            foreach (var i in query)
            {
                Console.WriteLine(i);
            }