LINQ Projection Operators

In LINQ (Language-Integrated Query), projection operators are used to select specific fields from a collection of objects, transforming the objects into a new form. This is typically done using the Select and SelectMany.

There are two types of Projection operators in LINQ.

1. Select
2. SlectMany

1. Select

The Select operator is used to project each element of a sequence into a new form. It works on a one-to-one basis, meaning each element in the source sequence is transformed into exactly one element in the result sequence.

Example

Suppose we have a list of employees and we want to project their first names and last names.

using System;
using System.Text;

namespace FirstProgram
{ 
    public class Employee
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var employees = new List<Employee>
            {
                  new Employee { FirstName = "Rohatash", LastName = "Kumar", Age = 35 },
                  new Employee { FirstName = "Mohit", LastName = "Kumar", Age = 32 },
                  new Employee { FirstName = "Saurav", LastName = "Kumar", Age = 34 }
            };

            // Using LINQ
            Console.WriteLine("Using LINQ Query");
            var result = from p in employees
                         select  new{FirstName = p.FirstName, LastName = p.LastName};

            foreach (var employee in result)
            {
                Console.WriteLine("First Name:" + employee.FirstName + ",Last Name:" + employee.LastName);
            }

            // Using lambda
            Console.WriteLine("");
            Console.WriteLine("Using lambda Method");
            var projectedData = employees.Select(e => new { e.FirstName, e.LastName });
            foreach (var employee1 in projectedData)
            {
                Console.WriteLine("First Name:" + employee1.FirstName+",Last Name:"+ employee1.LastName);
            }
            Console.ReadKey();
        }
    }
}

Output

2. SelectMany

The SelectMany operator is used to project each element of a sequence into a new form and then flatten the resulting sequences into one sequence. It works on a one-to-many basis, meaning each element in the source sequence can be transformed into zero or more elements in the result sequence.

Example

Suppose we have a list of departments, each with a list of employees, and we want to project all employees into a single flat list.

using System;
using System.Text;

namespace FirstProgram
{
    public class Department
    {
        public string Name { get; set; }
        public List Employees { get; set; }
    }
    public class Employee
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var departments = new List
           {
             new Department { Name = "HR", Employees = new List<Employee> { new Employee { FirstName = "Rohatash", LastName = "Kumar", Age = 35 } } },
                 new Department { Name = "IT", Employees = new List<Employee> {  new Employee { FirstName = "Mohit", LastName = "Kumar", Age = 32 } } },
                 new Department { Name = "IT", Employees = new List<Employee> {  new Employee { FirstName = "Saurav", LastName = "Kumar", Age = 34 } } }
           };
            // Using LINQ
            Console.WriteLine("Using LINQ Query");
            var allEmployees = from d in departments
                               from e in d.Employees
                               select e;


            foreach (var employee in allEmployees)
            {
                Console.WriteLine("First Name:" + employee.FirstName + ",First Name:" + employee.FirstName + ",Last Name:" + employee.LastName);
            }

            // Using lambda
            Console.WriteLine("");
            Console.WriteLine("Using lambda Method");
            var allEmployees1 = departments.SelectMany(d => d.Employees);
            foreach (var employee1 in allEmployees1)
            {
                Console.WriteLine("First Name:" + employee1.FirstName+",Last Name:"+ employee1.LastName);
            }
            Console.ReadKey();
        }
    }
}

Output

Key Differences

1. Projection Type

   • Select is used for a simple projection where each element in the source sequence maps to exactly one element in the result sequence.
   • SelectMany is used when each element in the source sequence can map to multiple elements in the result sequence, and the results need to be flattened into a single sequence.

2. Flattening

   • Select does not flatten sequences; it maintains the hierarchy of the source sequence.
   • SelectMany flattens the sequences, resulting in a single, flat sequence of elements.

3. Use Case

   • Use Select when you need to transform elements directly without changing the structure of the sequence.
   • Use SelectMany when you need to transform elements that result in nested sequences and you want a flat result.