Mệnh đề Select trong LINQ (C#)

Trong một biểu thức truy vấn, các mệnh đề select quy định các loại giá trị đó sẽ được trình bày khi truy vấn được thực hiện. Kết quả là dựa trên đánh giá của tất cả các mệnh đề trước và trên bất kỳ các biểu thức trong mệnh đề tự chọn. Một biểu thức truy vấn phải chấm dứt với cả một mệnh đề select hoặc một mệnh đề group.

Ví dụ sau đây cho thấy một mệnh đề select đơn giản trong một biểu thức truy vấn.

class SelectSample1
{
       static void Main()
       {
              //Create the data source
              List Scores = new List() { 97, 92, 81, 60 };


              // Create the query.
              IEnumerable queryHighScores = from score in Scores
                         where score < 80 select score;


             // Execute the query.
             foreach (int i in queryHighScores)
             {
                    Console.Write(i + " ");
             }
      }
}
//Output: 97 92 81

Ví dụ sau đây cho thấy tất cả các hình thức khác nhau mà một mệnh đề select có thể mất. Trong mỗi truy vấn, lưu ý các mối quan hệ giữa các mệnh đề select và loại biến truy vấn (studentQuery1, studentQuery2, vv).

class SelectSample2
{
        // Define some classes
        public class Student
        {
                 public string First { get; set; }
                 public string Last { get; set; }
                 public int ID { get; set; }
                 public List Scores;
                 public ContactInfo GetContactInfo(SelectSample2 app, int id)
                 {
                         ContactInfo cInfo = (from ci in app.contactList  where ci.ID == id select ci)
                                       .FirstOrDefault();
                         return cInfo;
                 }


                 public override string ToString()
                 {
                         return First + " " + Last + ":" + ID;
                 }
        }


        public class ContactInfo
        {
                public int ID { get; set; }
                public string Email { get; set; }
                public string Phone { get; set; }
                public override string ToString() { return Email + "," + Phone; }
         }


         public class ScoreInfo
         {
                 public double Average { get; set; }
                 public int ID { get; set; }
         }


         // The primary data source
         List students = new List()
         {
                  new Student {First="Svetlana", Last="Omelchenko", ID=111, Scores= new List() {97, 92, 81, 60}},
                  new Student {First="Claire", Last="O'Donnell", ID=112, Scores= new List() {75, 84, 91, 39}},
                  new Student {First="Sven", Last="Mortensen", ID=113, Scores= new List() {88,  94, 65, 91}},
                  new Student {First="Cesar", Last="Garcia", ID=114, Scores= new List() {97, 89, 85, 82}},
         };


         // Separate data source for contact info.
         List<ContactInfo> contactList = new List()
         {
                new ContactInfo {ID=111, Email="SvetlanO@Contoso.com", Phone="206-555-0108"},
                new ContactInfo {ID=112, Email="ClaireO@Contoso.com", Phone="206-555-0298"},
                new ContactInfo {ID=113, Email="SvenMort@Contoso.com", Phone="206-555-1130"},
                new ContactInfo {ID=114, Email="CesarGar@Contoso.com", Phone="206-555-0521"}
         };



         static void Main(string[] args)
         {
                  SelectSample2 app = new SelectSample2();
                  // Produce a filtered sequence of unmodified Students.
                  IEnumerable studentQuery1 = from student in app.students
                      where student.ID > 111 select student;


                  Console.WriteLine("Query1: select range_variable");
                  foreach (Student s in studentQuery1)
                  {
                          Console.WriteLine(s.ToString());
                   }


                   // Produce a filtered sequence of elements that contain
                  // only one property of each Student.
                  IEnumerable studentQuery2 =  from student in app.students
                       where student.ID > 111 select student.Last;


                  Console.WriteLine("\r\n studentQuery2: select range_variable.Property");
                  foreach (string s in studentQuery2)
                  {
                         Console.WriteLine(s);
                  }


                  // Produce a filtered sequence of objects created by
                  // a method call on each Student.
                  IEnumerable studentQuery3 = from student in app.students
                        where student.ID > 111  select student.GetContactInfo(app, student.ID);


                  Console.WriteLine("\r\n studentQuery3: select range_variable.Method");
                  foreach (ContactInfo ci in studentQuery3)
                  {
                         Console.WriteLine(ci.ToString());
                  }


                 // Produce a filtered sequence of ints from
                 // the internal array inside each Student.
                 IEnumerable studentQuery4 = from student in app.students
                        where student.ID > 111 select student.Scores[0];


                 Console.WriteLine("\r\n studentQuery4: select range_variable[index]");
                 foreach (int i in studentQuery4)
                 {
                            Console.WriteLine("First score = {0}", i);
                 }


                 // Produce a filtered sequence of doubles
                 // that are the result of an expression.
                 IEnumerable studentQuery5 = from student in app.students
                       where student.ID > 111 select student.Scores[0] * 1.1;


                 Console.WriteLine("\r\n studentQuery5: select expression");
                 foreach (double d in studentQuery5)
                 {
                         Console.WriteLine("Adjusted first score = {0}", d);
                 }


                 // Produce a filtered sequence of doubles that are
                 // the result of a method call.
                 IEnumerable studentQuery6 = from student in app.students
                        where student.ID > 111 select student.Scores.Average();


                 Console.WriteLine("\r\n studentQuery6: select expression2");
                 foreach (double d in studentQuery6)
                 {
                          Console.WriteLine("Average = {0}", d);
                 }


                // Produce a filtered sequence of anonymous types
                // that contain only two properties from each Student.
                var studentQuery7 = from student in app.students where student.ID > 111
                        select new { student.First, student.Last };


                Console.WriteLine("\r\n studentQuery7: select new anonymous type");
                foreach (var item in studentQuery7)
                {
                        Console.WriteLine("{0}, {1}", item.Last, item.First);
                 }


                 // Produce a filtered sequence of named objects that contain
                 // a method return value and a property from each Student.
                 // Use named types if you need to pass the query variable
                 // across a method boundary.
                 IEnumerable studentQuery8 = from student in app.students
                        where student.ID > 111 select new ScoreInfo
                        {
                               Average = student.Scores.Average(),
                               ID = student.ID
                        };


                 Console.WriteLine("\r\n studentQuery8: select new named type");
                 foreach (ScoreInfo si in studentQuery8)
                 {
                       Console.WriteLine("ID = {0}, Average = {1}", si.ID, si.Average);
                 }


                 // Produce a filtered sequence of students who appear on a contact list
                 // and whose average is greater than 85.
                IEnumerable studentQuery9 = from student in app.students
                        where student.Scores.Average() > 85 
                        join ci in app.contactList on student.ID equals ci.ID select ci;


                Console.WriteLine("\r\n studentQuery9: select result of join clause");
                foreach (ContactInfo ci in studentQuery9)
                {
                        Console.WriteLine("ID = {0}, Email = {1}", ci.ID, ci.Email);
                }


               // Keep the console window open in debug mode
               Console.WriteLine("Press any key to exit.");
               Console.ReadKey();
       }
}
/* Output
Query1: select range_variable
Claire O'Donnell:112
Sven Mortensen:113
Cesar Garcia:114


studentQuery2: select range_variable.Property
O'Donnell
Mortensen
Garcia


studentQuery3: select range_variable.Method
ClaireO@Contoso.com,206-555-0298
SvenMort@Contoso.com,206-555-1130
CesarGar@Contoso.com,206-555-0521


studentQuery4: select range_variable[index]
First score = 75
First score = 88
First score = 97


studentQuery5: select expression
Adjusted first score = 82.5
Adjusted first score = 96.8
Adjusted first score = 106.7


studentQuery6: select expression2
Average = 72.25
Average = 84.5
Average = 88.25


studentQuery7: select new anonymous type
O'Donnell, Claire
Mortensen, Sven
Garcia, Cesar


studentQuery8: select new named type
ID = 112, Average = 72.25
ID = 113, Average = 84.5
ID = 114, Average = 88.25


studentQuery9: select result of join clause
ID = 114, Email = CesarGar@Contoso.com
*/

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