LINQ projection operators( Select and SelectMany)

Introduction

Projection transforms the query result into the form defined by the developer. There are two projection operators in LINQ 

Let us say, there is a class 

And a function returning List<Student> as below, 

Select operator

Below query will return name and roll number of all the students. 

Output 

Below query will project name of the student’s starts with D. 

Output 

Above was very simple query let us modify the display function and try to display subject of the student 

Output 

If you notice the above output, we are not getting the proper output and it is saying that Subject is generic list. So to fetch we need to enumerate through the list.

Now question is how to retrieve all the subjects of students?

SelectMany

So to retrieve query from more than one collection SelectMany come into action. 

Above query will return the entire subjects of all the students.

Output 

The other way to apply SelectMany operator is directly apply on the retrieval query as below, 

Output 

Full Source code is as below,

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication9
{class Program
    {static void Main(string[] args)
        {// Reteriving all students with name D var result1 = from r in GetStudents()
                          where r.Name.StartsWith("D")
                          select r;foreach (var r in result1)
            {
                Console.WriteLine(r.Name);
            }// Reteriving  the result in Anonymous  classvar result2 = from r in GetStudents()
                          select new { r.RollNumber, r.Name };foreach (var r in result2)
            {
                Console.WriteLine(r);
            }// Reteriving using SelectMany var result3 = from r in GetStudents()
                          select r;foreach (var r inresult3.SelectMany(Student => Student.Subject))
            {
                Console.WriteLine(r);
            }// directly applying  SelectMany var result = GetStudents().AsQueryable().SelectMany(Subject => Subject.Subject);foreach (var r in result)
            {
                Console.WriteLine(r);
            }        
           
            Console.Read();
        }static List<Student> GetStudents()
        {
            List<Student> students = new List<Student>{new Student { 
                                         Name = "Dhananjay",
                                         RollNumber ="1" ,
                                         Subject= new List<string>{"Math","Phy"}},new Student { 
                                         Name = "Scott",
                                         RollNumber ="2" ,
                                         Subject= new List<string>{"Che","Phy"}},new Student { 
                                         Name = "John",
                                         RollNumber ="3" ,
                                         Subject= new List<string>{"Hindi","Phy"}}};return students; 
        }

    }class Student
    {public string Name { get; set; }public string RollNumber { get; set; }public List<string> Subject { get; set; }
    }
}

 I hope this article was useful. Thanks for reading. Happy Coding