pilih klausul (Referensi C#)
Dalam ekspresi kueri, klausul select menentukan jenis nilai yang akan dihasilkan saat kueri dijalankan. Hasilnya didasarkan pada evaluasi semua klausul sebelumnya dan pada ekspresi apa pun dalam klausul select itu sendiri. Ekspresi kueri harus dihentikan dengan klausul select atau klausul grup.
Contoh berikut menunjukkan klausul select sederhana dalam ekspresi kueri.
class SelectSample1
{
static void Main()
{
//Create the data source
List<int> Scores = [97, 92, 81, 60];
// Create the query.
IEnumerable<int> 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
Jenis urutan yang dihasilkan oleh klausul select menentukan jenis variabel kueri queryHighScores. Dalam kasus yang paling sederhana, klausul select hanya menentukan variabel rentang. Ini menyebabkan urutan yang dikembalikan berisi elemen dengan jenis yang sama dengan sumber data. Untuk informasi selengkapnya, lihat Mengetik Hubungan dalam Operasi Kueri LINQ. Namun, klausul select juga menyediakan mekanisme yang kuat untuk mengubah (atau memproyesikan) data sumber menjadi jenis baru. Untuk informasi selengkapnya, lihat Transformasi Data dengan LINQ (C#).
Contoh
Contoh berikut menunjukkan semua formulir berbeda yang mungkin diambil klausul select. Di setiap kueri, perhatikan hubungan antara klausul select dan jenis variabel kueri (studentQuery1, studentQuery2, dan sebagainya).
class SelectSample2
{
// Define some classes
public class Student
{
public required string First { get; init; }
public required string Last { get; init; }
public required int ID { get; init; }
public required List<int> 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() => $"{First} {Last}:{ID}";
}
public class ContactInfo
{
public required int ID { get; init; }
public required string Email { get; init; }
public required string Phone { get; init; }
public override string ToString() => $"{Email},{Phone}";
}
public class ScoreInfo
{
public double Average { get; init; }
public int ID { get; init; }
}
// The primary data source
List<Student> students =
[
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}},
new Student {First="Cesar", Last="Garcia", ID=114, Scores= new List<int>() {97, 89, 85, 82}},
];
// Separate data source for contact info.
List<ContactInfo> contactList =
[
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<Student> 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<String> 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<ContactInfo> 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<int> 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<double> 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<double> 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<ScoreInfo> 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<ContactInfo> 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);
}
}
}
/* 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
*/
Seperti yang ditunjukkan pada studentQuery8 dalam contoh sebelumnya, terkadang Anda mungkin ingin elemen urutan yang dikembalikan hanya berisi subset properti elemen sumber. Dengan menjaga urutan yang dikembalikan sekecil mungkin, Anda dapat mengurangi persyaratan memori dan meningkatkan kecepatan eksekusi kueri. Anda dapat menyelesaikan ini dengan membuat jenis anonim dalam klausul select dan menggunakan penginisialisasi objek untuk menginisialisasinya dengan properti yang sesuai dari elemen sumber. Untuk contoh cara melakukannya, lihat Penginisialisasi Objek dan Koleksi.
Keterangan
Pada waktu kompilasi, klausul select diterjemahkan ke panggilan metode ke operator kueri standar Select.
