LINQ query that filters elements from a list of object

Question

I have two lists of objects Person and PersonResult. Both are linked through the property PersonId. I need to create a filter for the list of PersonResult that meet certain criteria for the Person (e.g. Person.Gender == "female").

Im currently using the following LINQ query to achieve this:

PersonResultList = PersonResultList.Where(pr => 
    PersonList.FirstOrDefault(p => pr.PersonId == p.PersonId) != null && 
    PersonList.FirstOrDefault(p => pr.PersonId == p.PersonId).Gender == "female");

This works apparently well, however, I must iterate twice through PersonList to check if the person exist and its gender. Is there a more elegant way to achieve this?

Answer 1

You can simply combine the conditions inside the FirstOrDefault() like

PersonResultList = PersonResultList
    .Where(pr => PersonList
                    .FirstOrDefault(p => pr.PersonId == p.PersonId 
                                         && p.Gender == "female") != null );  

Because I only changed your existing code, it didn't came to my mind what Nikita Brizhak commented here .

You should probably use Any instead of FirstOrDefault

So let us change the code to

PersonResultList = PersonResultList
        .Where(pr => PersonList
                        .Any(p => pr.PersonId == p.PersonId 
                              && p.Gender == "female")); 

This is based on the assumption that for each entry in the first list there will be only one entry in the second list.

Answer 2

What you are doing is filtering PersonResultList to only include female genders. Joining to add information from PersonList expresses this more clearly and efficiently.

Using extension methods:

PersonResultList = PersonResultList.Join(
    PersonList.Where(_ => _.Gender == "female"), 
    personResult => personResult.PersonId,
    person => person.PersonId, 
    (personResult, person) => personResult);

Pretty much the same thing, but clearer:

PersonResultList = from personResult in PersonResultList
                   join person in PersonList on personResult.PersonId equals person.PersonId
                   where person.Gender == "female"
                   select personResult;