select 子句

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在查询表达式中，select 子句可以指定将在执行查询时产生的值的类型。该子句的结果将基于前面所有子句的计算结果以及 select 子句本身中的所有表达式。查询表达式必须以 select 子句或 group 子句结束。

下面的示例演示了查询表达式中的简单 select 子句。

C#	复制代码
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

复制代码

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 子句产生的序列的类型决定了查询变量 queryHighScores 的类型。在最简单的情况下，select 子句仅指定范围变量。这会使返回的序列包含与数据源具有相同类型的元素。有关更多信息，请参见查询操作中的类型关系 (LINQ)。不过，select 子句还提供了一种功能强大的机制，可用于将源数据转换（或投影）为新类型。有关更多信息，请参见使用 LINQ 进行数据转换。

示例

下面的示例演示了 select 子句可能采用的所有不同形式。在每个查询中，请注意 select 子句和查询变量（studentQuery1、studentQuery2 等）的类型之间的关系。

C#	复制代码
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 */

复制代码

    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
*/

如上一个示例中的 studentQuery8 所示，您有时可能希望所返回序列中的元素仅包含源元素的属性子集。通过使返回的序列尽可能地小一些，可以降低内存需求，并提高查询的执行速度。通过在 select 子句中创建一个匿名类型，并且借助于对象初始值设定项用源元素中的适当属性对该匿名类型进行初始化，可以达到此目的。有关如何执行此操作的示例，请参见对象和集合初始值设定项（C# 编程指南）。