Does in Linq where creates a new instance of object

Question

In below code why filteredStudentNames holds only 2 elements. As per my understanding, it is reference type so when I update the studentList object with one more item that results should also be part of filteredStudentNames.

In the case of studentNames object, I am getting the updated record also. So why not in the case of filteredStudentnames?

static void Main(string[] args)
    {
        IList<Student> studentList = new List<Student>() {
                    new Student() { StudentID = 1, StudentName = "John", age = 18 } ,
                    new Student() { StudentID = 2, StudentName = "Steve",  age = 21 } ,
                    new Student() { StudentID = 3, StudentName = "Bill",  age = 18 } ,
                    new Student() { StudentID = 4, StudentName = "Ram" , age = 20  } ,
                    new Student() { StudentID = 5, StudentName = "Ron" , age = 21 }
                };

        var filteredStudentNames = from s in studentList 
                                   where s.age > 20
                                   select s;

        var studentNames = from s in studentList
                           select s;

        IList<Student> stud = studentList;
        studentList.Add(new Student { StudentID = 27, StudentName = "Test" });

Your `filteredStudentNames` is queried for `s.age > 20` and so it won't exist — Hary, Oct 31 '18 at 10:54
LINQ uses deferred execution. Created queries are only executed when you do `foreach` on them or use another execution techniques (like `ToList` or `FirstOrDefault`). — dymanoid, Oct 31 '18 at 10:54
Also you are not adding new student with age>20. Sid you try that? — Chetan, Oct 31 '18 at 10:56
Probably a typo- you meant to write `studentList.Add(new Student { StudentID = 6, StudentName = "Test", age=27 });` — Ofir Winegarten, Oct 31 '18 at 10:57
Ofir is right, that's just a typo question and can be closed. If OP had a real world issue he needs to show the real code. — Tim Schmelter, Oct 31 '18 at 11:07

Hary · Accepted Answer · 2018-10-31T11:08:51.963

1

filteredStudentNames is queried with condition age > 20. On adding new Student you have not initiated the age property which means by default it is 0.

So it is not reflecting in filteredStudentNames

Try initiating the age property and then query the Count

studentList.Add(new Student { StudentID = 6, StudentName = "Test", age = 21 });

Count:

Console.WriteLine("filteredStudentNames: " + filteredStudentNames.Count()); //Output: 3
Console.WriteLine("studentNames: " + studentNames.Count());   //Output: 6

edited Oct 31 '18 at 11:08

answered Oct 31 '18 at 11:02

Hary

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`StudentID = 27` is probably a typo, the ID should be `6` in this example and the age `27`. – Tim Schmelter Oct 31 '18 at 11:06
@TimSchmelter, Yes. it should be, I have just re-used the example as in the question. – Hary Oct 31 '18 at 11:08
1

@mbharanidharan88 Thanks.. it was a typo.. silly mistake. – Saurabh Oct 31 '18 at 14:06

Thili77 · Answer 2 · 2018-10-31T10:59:09.973

I'm pretty sure this will be work for you.

static void Main(string[] args)
{
    IList<Student> studentList = new List<Student>() {
                new Student() { StudentID = 1, StudentName = "John", age = 18 } ,
                new Student() { StudentID = 2, StudentName = "Steve",  age = 21 } ,
                new Student() { StudentID = 3, StudentName = "Bill",  age = 18 } ,
                new Student() { StudentID = 4, StudentName = "Ram" , age = 20  } ,
                new Student() { StudentID = 5, StudentName = "Ron" , age = 21 }
            };

    var filteredStudentNames = from s in studentList where s.age > 20
                       select s;

    var studentNames = from s in studentList
                       select s;

    IList<Student> stud = studentList;
    studentList.Add(new Student { StudentID = 27, StudentName = "Test", age = 25 });


    var filteredStudentNames = from s in studentList where s.age > 20
                       select s;

}

You have to execute link after the adding by new entry and you have to add age for the new entry. Thanks.

`You have to execute link after the adding by new entry.` No you don't. https://stackoverflow.com/questions/7324033/what-are-the-benefits-of-a-deferred-execution-in-linq Also, your code does not compile. — mjwills, Oct 31 '18 at 11:02

Does in Linq where creates a new instance of object

2 Answers2