Linq Comparing Two Lists and Generating a New One

Question

I am Working on a MVC4 project and I am in need to create a new list of objects by comparing two lists. db below refers to the DbContext object.

In the project that I work, I am in need of comparing two lists. The first has all the subjects offered in a semester. The other has the subjects selected by the student.

List<Subject> subjects = db.Subjects;
//...
List<Subject> semesterSubjects = student.Subjects;

Out of this two lists I need to create a list of items that state if or not each an every item is selected. It looks like this.

class SubjectSelection
{
public int ID { get; set; }
public String Title { get; set; }
public bool IsSelected { get; set; }
}

What I need now is to compare subjects & semesterSubjects and get a IEnumerable<SubjectSelection>

Could I use Linq to get such a result and how?

Answer 1

Best way is to do this job on the server side. Select ids from your semester subjects:

var selectedIDs = semesterSubjects.Select(s => s.ID).ToList();

And pass them to query:

var query = db.Subjects.Select(s => new SubjectSelection()
                     {
                        ID = s.ID,
                        Title = s.Title,
                        IsSelected = selectedIDs.Contains(s.ID)
                     });

If you are using same db context instance to retrieve both semester subjects list and subjects list, then you can simply use:

var query = subjects.Select(s => new SubjectSelection()
                     {
                        ID = s.ID,
                        Title = s.Title,
                        IsSelected = semesterSubjects.Contains(s)
                     });

Entity Framework implements identity map pattern, which means it returns same in-memory instance of subject object, which returned by different queries.

Answer 2

var allSubjectsWithSelection = subjects.Select(o=>new SubjectSelection(){ID = o.ID,Title = o.Title, IsSelected = semesterSubjects.Any(s=>s.ID == o.ID)}).ToList();