Part 7 - Projection Operators in LINQ

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The following 2 standard LINQ query operators belong to Projection Operators category.
Select
SelectMany



Projection Operators (Select & SelectMany) are used to transform the results of a query. In this video we will discuss Select operator and in a later video session we will discuss SelectMany operator.

Select clause in SQL allows to specify what columns we want to retrieve. In a similar fashion LINQ SELECT standard query operator allows us to specify what properties we want to retrieve. It also allows us to perform calculations.

For example, you may have a collection of Employee objects. The following are the properties of the Employee class.
EmployeeID
FirstName
LastName
AnnualSalay
Gender

Now using the SELECT projection operator
1. We can select just EmployeeID property OR
2. We can select multiple properties (FirstName & Gender) into an anonymous type OR
3. Perform calculations 
    a) MonthlySalary = AnnualSalay/12
    b) FullName = FirstName + " " + LastName

We will be using the following Employee class for this demo.

public class Employee

{

    public int EmployeeID { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public string Gender { get; set; }

    public int AnnualSalary { get; set; }

    public static List<Employee> GetAllEmployees()

    {

        List<Employee> listEmployees = new List<Employee>

        {

            new Employee

            {

                EmployeeID = 101,

                FirstName = "Tom",

                LastName = "Daely",

                Gender = "Male",

                AnnualSalary = 60000

            },

            new Employee

            {

                EmployeeID = 102,

                FirstName = "Mike",

                LastName = "Mist",

                Gender = "Male",

                AnnualSalary = 72000

            },

            new Employee

            {

                EmployeeID = 103,

                FirstName = "Mary",

                LastName = "Lambeth",

                Gender = "Female",

                AnnualSalary = 48000

            },

            new Employee

            {

                EmployeeID = 104,

                FirstName = "Pam",

                LastName = "Penny",

                Gender = "Female",

                AnnualSalary = 84000

            },

        };

        return listEmployees;

    }

}

Example 1: Retrieves just the EmployeeID property of all employees

IEnumerable<int> employeeIds = Employee.GetAllEmployees()

    .Select(emp => emp.EmployeeID);

foreach (int id in employeeIds)

{

    Console.WriteLine(id);

}

Output:


Example 2: Projects FirstName & Gender properties of all employees into anonymous type.

var result = Employee.GetAllEmployees().Select(emp => new

                    {

                        FirstName = emp.FirstName,

                        Gender = emp.Gender

                    });

foreach (var v in result)

{

    Console.WriteLine(v.FirstName + " - " + v.Gender);

}

Output:


Example 3: Computes FullName and MonthlySalay of all employees and projects these 2 new computed properties into anonymous type.

var result = Employee.GetAllEmployees().Select(emp => new

{

    FullName = emp.FirstName + " " + emp.LastName,

    MonthlySalary = emp.AnnualSalary / 12

});

foreach (var v in result)

{

    Console.WriteLine(v.FullName + " - " + v.MonthlySalary);

}

Output:


Example 4: Give 10% bonus to all employees whose annual salary is greater than 50000 and project all such employee's FirstName, AnnualSalay and Bonus into anonymous type.

var result = Employee.GetAllEmployees()

                .Where(emp => emp.AnnualSalary > 50000)

                .Select(emp => new

                 {

                    Name = emp.FirstName,

                    Salary = emp.AnnualSalary,

                    Bonus = emp.AnnualSalary * .1

                 });

foreach (var v in result)

{

    Console.WriteLine(v.Name + " : " + v.Salary + " - " + v.Bonus);

}

Output: