LINQ | Projection Operator | SelectMany

In LINQ, projection is an operation which converts an object into the new form which holds only those properties which will be subsequently used. By using projection, a developer can create a new type which is built from each object. You are allowed to project property and conduct mathematical function on it, and you can also project the original object without transforming it.

In LINQ, the following projection operations are available:

1. Select
2. SelectMany

SelectMany Operator

The SelectMany operator returns sequences of values which are based on the transformation function and then make them into one sequence. Or in other words, we can say, SelectMany operator is used when you want to select values from the multiple collections or if you want a result from the list of the lists and wants to display into a single sequence.

SelectMany in Query Syntax: In Query Syntax, the working of SelectMany operator is achieved by using multiple from clause. As shown in the below example.

Example:

// C# program to find the languages
// known by the employee
using System;
using System.Linq;
using System.Collections.Generic;

// Employee details
public class Employee {

    public int emp_id
    {
        get;
        set;
    }
    public string emp_name
    {
        get;
        set;
    }
    public string emp_gender
    {
        get;
        set;
    }
    public string emp_hire_date
    {
        get;
        set;
    }
    public int emp_salary
    {
        get;
        set;
    }
    public List<string> emp_lang
    {
        get;
        set;
    }
}

class GFG {

    // Main method
    static public void Main()
    {
        List<Employee> emp = new List<Employee>() {
            new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female", emp_hire_date = "12/3/2017", emp_salary = 20000, emp_lang = new List<string>{"C#", "VB"} },
                new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female", emp_hire_date = "22/4/2018", emp_salary = 30000,  emp_lang = new List<string>{ "Java"} },
                new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male", emp_hire_date = "3/5/2016", emp_salary = 40000,  emp_lang = new List<string>{ "C++", "SQL"} },
                new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female", emp_hire_date = "4/8/2017", emp_salary = 40000,  emp_lang = new List<string>{"Python", "C", "PHP"} },
                new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male", emp_hire_date = "12/1/2016", emp_salary = 40000,  emp_lang = new List<string>{"HTML", "JQuery"} },
                new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female", emp_hire_date = "17/6/2015", emp_salary = 50000,  emp_lang = new List<string>{"JavaScript", "Perl"} },
        };

        // Query to find the languages
        // known by the employee
        var res = from e in emp
                      from e2 in e.emp_lang
                          select e2;

        Console.WriteLine("Languages known by all the employees are:");
        foreach(var val in res)
        {
            Console.WriteLine(val);
        }
    }
}

Output:

Languages known by all the employees are:
C#
VB
Java
C++
SQL
Python
C
PHP
HTML
JQuery
JavaScript
Perl

SelectMany in Method Syntax: The SelectMany method is present in both the Queryable and Enumerable class and supported by both C# and VB.Net languages.

Example:

// C# program to find the languages
// known by the employee
using System;
using System.Linq;
using System.Collections.Generic;

// Employee details
public class Employee {

    public int emp_id
    {
        get;
        set;
    }
    public string emp_name
    {
        get;
        set;
    }
    public string emp_gender
    {
        get;
        set;
    }
    public string emp_hire_date
    {
        get;
        set;
    }
    public int emp_salary
    {
        get;
        set;
    }
    public List<string> emp_lang
    {
        get;
        set;
    }
}

public class GFG {

    // Main method
    static public void Main()
    {
        List<Employee> emp = new List<Employee>() {
            new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female", emp_hire_date = "12/3/2017", emp_salary = 20000, emp_lang = new List<string>{"C#", "VB"} },
                new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female", emp_hire_date = "22/4/2018", emp_salary = 30000,  emp_lang = new List<string>{ "Java"} },
                new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male", emp_hire_date = "3/5/2016", emp_salary = 40000,  emp_lang = new List<string>{ "C++", "SQL"} },
                new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female", emp_hire_date = "4/8/2017", emp_salary = 40000,  emp_lang = new List<string>{"Python", "C", "PHP"} },
                new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male", emp_hire_date = "12/1/2016", emp_salary = 40000,  emp_lang = new List<string>{"HTML", "JQuery"} },
                new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female", emp_hire_date = "17/6/2015", emp_salary = 50000,  emp_lang = new List<string>{"JavaScript", "Perl"} },
        };

        // Finding the languages known by the employee
        // Using SelectMany method
        var res = emp.SelectMany(a => a.emp_lang);
        Console.WriteLine("Languages known by all the employees are:");
        foreach(var val in res)
        {
            Console.WriteLine(val);
        }
    }
}

Output:

Languages known by all the employees are:
C#
VB
Java
C++
SQL
Python
C
PHP
HTML
JQuery
JavaScript
Perl