lambda extension to combine lists

Question

how do i use the second expression to select only those with ID from the first?

 var list1= from x in objects select x.id;


 results=results.Where(r=>r.id==  ????  )

I want the results to be only those with id from listA

tia

EDIT:i stand corrected, there was another issue causing problem which i will ask about separately.

Answer 1

Like so...

results.Where(r=>list1.Contains(r.id))

Answer 2

results = results.Where(r => list1.Contains(r.id));

Answer 3

If you want some performance (list.Contains() has an O(n) complexity) you could go with

var ids = objects.ToDictionary(o => o.id);

results.Where(o => ids.ContainsKey(o.id));

Answer 4

Maybe you want this then?

results.Where(r=> objects.Any(o => o.id == r.id) )

Answer 5

A bit of a guess (haven't tried running it), but:

var filteredResults = from obj in objects 
                      join result in results on obj.id equals result.id
                      select result;

Note that this should replace both the lines of code you have in your question.

Answer 6

If You always need only the first element's ID , You can store it to variable and use it to lambda expression

var results= from x in objects select x.id;
int firstID = results.First().id ;
results=results.Where(r=>r.id==  firstID  )

Or, use directly like this:

var results= from x in objects select x.id;
results=results.Where(r=>r.id==  results.First().id  )