Home How do I used LINQ to retrieve a distinct list of object X from a list of object Y where object X is stored in a list against object Y

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How do I used LINQ to retrieve a distinct list of object X from a list of object Y where object X is stored in a list against object Y

byMR

January 19, 2024

I have a list of objects ("Forms") and each of those objects has it’s own nested list of another object ("Subjects").

My Form object looks like this:

public class Form 
{
    public int Id { get; set; }
    public IEnumerable<Subject>? LinkedSubjects { get; set; }
}

And my nested Subject object looks like this:

public class Subject 
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Let’s say I return a list of forms, each of which has its own nested list of linked subjects. One form might be linked to subjects with IDs of 1 and 2, another might be linked to subjects with IDs of 2 and 3, etc.

Using LINQ, how do I select the lists of subjects from this list of forms, combine them, and then remove any duplicates to leave me with a list of distinct subjects?

Thanks!

>Solution :

Not tested, self-explanatory:

var distinctSubjects =
   listOfForms
   .SelectMany( x => x.LinkedSubjects ) // list of subjects
   .GroupBy( x => x.Id ) // grouping by id
   .Select( x => x.First() ) // get first subject from each group

Why to group instead use Distinct ? Because I don’t know if, in your code, is really the same Subject on two forms (if yes, just use Distinct) or is a different object with the same values.