How do I perform a query with a list in C#

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Hi, I just was wondering if I can perform a Query just like that:

What I have tried:

C#

List <studentclasslist> = _context.StudentClass
                    .Where(sc => sc.ClassId == 1)
                    .ToList();
 
List<int> studentIdList = new List<int>(); 

foreach (var student in studentClassList )
{
   studentIdList = _context.Student.Where(st => st.Id == student.StudentId).ToList(); 
}

Thanks in advance.

Posted 6-May-19 6:48am

Member 14087498

Updated 7-May-19 1:34am

Maciej Los

v2

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BillWoodruff · Answer 1 · 2019-05-07T01:35:00

To me this kind of code ... that will obviously not compile ... indicates you have not really studied the basics of using the powerful tools that Linq provides.

Don't worry, though: everyone I know has gone through a learning curve to get somewhat fluent in using Linq :) Well, okay, imho, people with a deep background in DB query languages have an easier time; I just don't know them :)

Please consider getting a book on Linq, and using the on-line tool, LinqPad [^], provided by the Albihari brothers. They have several books out on Linq; and, you may find this useful: [^].

I have found my students respond well to the idea that the 'Where operator is a "selection function;" and, the 'Select operator is a function that enumerates and transforms elements into another form.

Christian Graus · Answer 2 · 2019-05-06T12:32:00

Solution 2

Just to add, you need to include the Linq namespace for that stuff to start working, otherwise you'll have a List class but not those operators

List<int> studentIdList = new List<int>();

Also, this makes no sense. Set it to null if you need to set it inside a scope. Your code sets it over and over and does nothing with it? In fact, your code won't compile. Post real code when you ask a question....

Posted 6-May-19 12:32pm

Christian Graus

Updated 6-May-19 12:33pm

v2

Comments

BillWoodruff 7-May-19 18:35pm

+4 to counteract an undeserved down-vote

Christian Graus 7-May-19 18:39pm

Cheers

OriginalGriff · Answer 3 · 2019-05-06T06:58:00

Well, you could ... if you got the names right, C# is case sensitive ... but it wouldn't do much that was useful.

The assignment inside the loop means that the eventual result will be the final result only, because it "throws away" all previous results each time round the loop.

You might want to consider List.AddRange instead of an assignment, but at a guess you might want to think about your data and the relationships between them rather than leaping into code like that - even without knowing exactly what you are trying to do that looks like a very inefficient way to do things!

Maciej Los · Answer 4 · 2019-05-06T22:13:00

Solution 3

This piece of code is not clear to me:

C#

List<int> studentIdList = new List<int>();

foreach (var student in studentClassList )
{
studentIdList = _context.Student.Where(st => st.Id == student.StudentId).ToList();
}

If you would like to get list of students in a specific class, you should do something like this:

C#

var studentsinclass = _context.Student
    .Where(st => _context.StudentClass
        .Any(sc => sc.ClassId == 1 && st.Id == sc.StudentId))
    .ToList();

Good luck!

Posted 6-May-19 22:13pm

Maciej Los

v2

Comments

BillWoodruff 7-May-19 18:34pm

Hi Maciej, the use of 'Any in your last example will return a bool if any element matches ... I don't think that's what you really want here.

cheers, Bill

Maciej Los 8-May-19 2:08am

Bill, take a look at example again. 'Any' method is used as a condition to 'Where'.
Anyway, thanks for your comment.
Cheers,
Maciej

BillWoodruff 8-May-19 3:01am

Okay, I am re-calibrating ... thanks !

How do I perform a query with a list in C#

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