I have 2 lists of different objects (foo & bar) that share the same property lets call it id.
public List<foo> foo { get; set; }
public List<bar> bar { get; set; }
I want to remove all objects from foo that have an id that does not exist in bar
How can this be done in linq? I have been looking at Intersect, RemoveAll & Join but cannot find any example where the lists are of a different type.
Try this:
foo.RemoveAll(x=> !bar.Any(y=>y.Id==x.Id));
!bar.Any(y=>y.Id==x.Id) will get if item is in bar collection and if it's not it will remove it from foo collection.
Better solution using hashset O(n):
var idsNotToBeRemoved = new HashSet<int>(bar.Select(item => item.Id));
foo.RemoveAll(item => !idsNotToBeRemoved.Contains(item.Id));
source of second answer: https://stackoverflow.com/a/4037674/1714342
EDIT:
as @Carra said, first solution is good for small lists and second is more efficient for big lists.
var foo = foo.Where(f => !bar.Any(b => b.Id == f.Id)).ToList();
Just keep in mind that this is a O(n²) solution, it won't work very well for big lists.
ids from bar into a hashset (O(n)) then compare each element in foo to that (O(n)) rather than walking the bar list for every element in foonew HashSet<int>(bar.Select(x => x.id)) ought to do, assuming you have no collisions