select 子句 (C# 參考)
在查詢運算式中，select 子句會指定執行查詢時，將產生的實值型別。 結果是根據所有先前子句的評估和 select 子句本身的任何運算式而得。 查詢運算式必須終止 select 子句或 group 子句。
下列範例顯示查詢運算式中簡單的 select 子句。
class SelectSample1
{
static void Main()
{
//Create the data source
List<int> Scores = new List<int>() { 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
select 子句產生的序列型別會決定查詢變數 queryHighScores 的型別。 在最簡單的情況下，select 子句只要指定範圍變數。 這會使傳回的序列包含與資料來源相同型別的項目。 如需詳細資訊，請參閱LINQ 查詢作業中的型別關聯性 (C#)。 但是，select 子句也提供功能強大的機制，可將來源資料轉換 (或「投影」(Projecting)) 為新型別。 如需詳細資訊，請參閱 使用 LINQ 轉換資料 (C#)。
範例
下列範例顯示 select 子句可能會採用的不同格式。 在每個查詢中，請注意 select 子句與「查詢變數」(Query Variable) (studentQuery1、studentQuery2 等等) 之型別之間的關係。
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<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()
{
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<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}},
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 List<ContactInfo>()
{
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);
}
// 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
*/
如同前述範例中的 studentQuery8 所示，有時您可能想要傳回之序列的項目只包含來源項目之屬性的子集。 盡可能使傳回的序列越小，您就可以減少記憶體需求，並且增加執行查詢的速度。 您可以藉由在 select 子句中建立匿名型別，以及使用物件初始設定式以來源項目的適當屬性進行初始化，達成這個目的。 如需如何進行的範例，請參閱物件和集合初始設定式 (C# 程式設計手冊)。
備註
在編譯時期，select 子句會轉譯成對 Select 標準查詢運算子的方法呼叫。