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.
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 class
var 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 in
result3.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
Dhananjay Kumar 
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