LINQ 查询操作及进行数据转换

由于已熟悉查询语言（如 SQL 或 XQuery），基本的查询操作不再叙述。因为类似。

此外

例如，可以执行下列任务：

• 将多个输入序列合并到具有新类型的单个输出序列中。

• 创建其元素只包含源序列中的各个元素的一个或几个属性的输出序列。

• 创建其元素包含对源数据执行的操作结果的输出序列。

• 例如，您可以将 SQL 行或文本文件的数据转换为 XML。

将多个输入联接到一个输出序列

---

假定下面两种类类型：

    class Student
    {
        public string First { get; set; }
        public string Last { get; set; }
        public int ID { get; set; }
        public string Street { get; set; }
        public string City { get; set; }
        public List<int> Scores;
    }

    class Teacher
    {
        public string First { get; set; }
        public string Last { get; set; }
        public int ID { get; set; }
        public string City { get; set; }
    }
    class DataTransformations
    {
        static void Main()
        {
            // Create the first data source.
            List<Student> students = new List<Student>()
        {
            new Student {First="Svetlana",
                Last="Omelchenko", 
                ID=111, 
                Street="123 Main Street",
                City="Seattle",
                Scores= new List<int> {97, 92, 81, 60}},
            new Student {First="Claire",
                Last="O’Donnell", 
                ID=112,
                Street="124 Main Street",
                City="Redmond",
                Scores= new List<int> {75, 84, 91, 39}},
            new Student {First="Sven",
                Last="Mortensen",
                ID=113,
                Street="125 Main Street",
                City="Lake City",
                Scores= new List<int> {88, 94, 65, 91}},
        };

            // Create the second data source.
            List<Teacher> teachers = new List<Teacher>()
        {                
            new Teacher {First="Ann", Last="Beebe", ID=945, City = "Seattle"},
            new Teacher {First="Alex", Last="Robinson", ID=956, City = "Redmond"},
            new Teacher {First="Michiyo", Last="Sato", ID=972, City = "Tacoma"}
        };

            // Create the query.
            var peopleInSeattle = (from student in students
                                   where student.City == "Seattle"
                                   select student.Last)
                        .Concat(from teacher in teachers
                                where teacher.City == "Seattle"
                                select teacher.Last);

            Console.WriteLine("The following students and teachers live in Seattle:");
            // Execute the query.
            foreach (var person in peopleInSeattle)
            {
                Console.WriteLine(person);
            }

            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
        }
    }

输出结果：

The following students and teachers live in Seattle:
    Omelchenko
    Beebe

选择各个源元素的子集

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选择源序列中的各个元素的子集有两种主要方法：

1. 在执行此查询时，此查询将生成字符串输出序列。

    

    

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

2. Customer 元素的两个属性：

    

    

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

将内存中的对象转换为 XML

---

下面的示例将内存中的数据结构中的对象转换为 XML 元素。

    class Student
    {
        public string First { get; set; }
        public string Last { get; set; }
        public int ID { get; set; }
        public string Street { get; set; }
        public string City { get; set; }
        public List<int> Scores;
    }

    class Teacher
    {
        public string First { get; set; }
        public string Last { get; set; }
        public int ID { get; set; }
        public string City { get; set; }
    }
    class XMLTransform
    {
        static void Main()
        {
            // Create the data source by using a collection initializer.
            List<Student> students = new List<Student>()
        {
            new Student {First="Svetlana", Last="Omelchenko", ID=111, Scores = new List<int>{97, 92, 81, 60}},
            new Student {First="Claire", Last="O’Donnell", ID=112, Scores = new List<int>{75, 84, 91, 39}},
            new Student {First="Sven", Last="Mortensen", ID=113, Scores = new List<int>{88, 94, 65, 91}},
        };

            // Create the query.
            var studentsToXML = new XElement("Root",
                from student in students
                let x = String.Format("{0},{1},{2},{3}", student.Scores[0],
                        student.Scores[1], student.Scores[2], student.Scores[3])
                select new XElement("student",
                           new XElement("First", student.First),
                           new XElement("Last", student.Last),
                           new XElement("Scores", x)
                        ) // end "student"
                    ); // end "Root"

            // Execute the query.
            Console.WriteLine(studentsToXML);

            // Keep the console open in debug mode.
            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
        }
    }

此代码生成下面的 XML 输出：

<Root>
  <student>
    <First>Svetlana</First>
    <Last>Omelchenko</Last>
    <Scores>97,92,81,60</Scores>
  </student>
  <student>
    <First>Claire</First>
    <Last>O’Donnell</Last>
    <Scores>75,84,91,39</Scores>
  </student>
  <student>
    <First>Sven</First>
    <Last>Mortensen</Last>
    <Scores>88,94,65,91</Scores>
  </student>
</Root>
Press any key to exit.

对源元素执行操作

---

double 类型的元素的源序列进行的计算。

class FormatQuery
{
    static void Main()
    {            
        // Data source.
        double[] radii = { 1, 2, 3 };

        // Query.
        IEnumerable<string> query =
            from rad in radii
            select String.Format("Area = {0}", (rad * rad) * 3.14);

        // Query execution. 
        foreach (string s in query)
            Console.WriteLine(s);

        // Keep the console open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output:
    Area = 3.14
    Area = 12.56
    Area = 28.26
*/