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select clause (C# Reference)

In a query expression, the select clause specifies the type of values that will be produced when the query is executed. The result is based on the evaluation of all the previous clauses and on any expressions in the select clause itself. A query expression must terminate with either a select clause or a group clause.

The following example shows a simple select clause in a query expression.


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


The type of the sequence produced by the select clause determines the type of the query variable queryHighScores. In the simplest case, the select clause just specifies the range variable. This causes the returned sequence to contain elements of the same type as the data source. For more information, see Type Relationships in LINQ Query Operations (C#). However, the select clause also provides a powerful mechanism for transforming (or projecting) source data into new types. For more information, see Data Transformations with LINQ (C#).

The following example shows all the different forms that a select clause may take. In each query, note the relationship between the select clause and the type of the query variable (studentQuery1, studentQuery2, and so on).


    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
*/


As shown in studentQuery8 in the previous example, sometimes you might want the elements of the returned sequence to contain only a subset of the properties of the source elements. By keeping the returned sequence as small as possible you can reduce the memory requirements and increase the speed of the execution of the query. You can accomplish this by creating an anonymous type in the select clause and using an object initializer to initialize it with the appropriate properties from the source element. For an example of how to do this, see Object and Collection Initializers (C# Programming Guide).

At compile time, the select clause is translated to a method call to the Select standard query operator.

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