Projection Operators: Select, SelectMany

There are two projection operators available in LINQ. 1) Select 2) SelectMany

Select:

The Select operator always returns an IEnumerable collection which contains elements based on a transformation function. It is similar to the Select clause of SQL that produces a flat result set.

Now, let's understand Select query operator using the following Student class.

Example Classes

public class Student{ 
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public int Age { get; set; }
}

Select in Query Syntax:

LINQ query syntax must end with a Select or GroupBy clause. The following example demonstrates select operator that returns a string collection of StudentName.

Example: Select in query syntax C#

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John" },
    new Student() { StudentID = 2, StudentName = "Moin" },
    new Student() { StudentID = 3, StudentName = "Bill" },
    new Student() { StudentID = 4, StudentName = "Ram" },
    new Student() { StudentID = 5, StudentName = "Ron" } 
};

var selectResult = from s in studentList
                   select s.StudentName; 

The select operator can be used to formulat the result as per our requirement. It can be used to return a collection of custom class or anonymous type which includes properties as per our need.

The following example of the select clause returns a collection of anonymous type containing the Name and Age property.

Example: Select operator in query syntax C#

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 13 } ,
    new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
};

// returns collection of anonymous objects with Name and Age property
var selectResult = from s in studentList
                   select new { Name = "Mr. " + s.StudentName, Age = s.Age }; 

// iterate selectResult
foreach (var item in selectResult)
    Console.WriteLine("Student Name: {0}, Age: {1}", item.Name, item.Age);

Example: Select operator in query syntax VB.Net

Dim selectResult = From s In studentList
                   Select New With {.Name = s.StudentName, .Age = s.Age}

Output:
Student Name: Mr. John, Age: 13
Student Name: Mr. Moin, Age: 21
Student Name: Mr. Bill, Age: 18
Student Name: Mr. Ram, Age: 20
Student Name: Mr. Ron, Age: 15

Select in Method Syntax:

The Select operator is optional in method syntax. However, you can use it to shape the data. In the following example, Select extension method returns a collection of anonymous object with the Name and Age property:

Example: Select in method syntax C#

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 21 } 
};
    
var selectResult = studentList.Select(s => new { Name = s.StudentName , 
                                                 Age = s.Age  });

In the above example, selectResult would contain anonymous objects with Name and Age property as shown below in the debug view.

Select clause returns an Anonymous objects
Example: Select in method syntax VB.Net

Dim selectResult = studentList.Select(Function(s) New With {.Name = s.StudentName,
                                                            .Age = s.Age})

Select Many:

The SelectMany operator projects sequences of values that are based on a transform function and then flattens them into one sequence.

Visit MSDN for more information on projection operators.