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