List of queries with lambda linq expressions

Question

How do I get members who are on the county list? I get counties in var counties, then I want to get all the members who have CountyOfParticipationId, which is on the counties list.

if (collaborationId != null) { var counties = (from c in db.CountyCollaborations where c.CollaborationId == collaborationId select c).ToList(); participants = participants.Where(p => p.CountyOfParticipationId in counties); } 

Answer 1

.Where(p => counties.Contains(p.CountyOfParticipationId))

Now, if there is a lot of data, be careful with the complexity of this. Contains in the list O (n), therefore, in general, the algorithm is O (n * m), where n, m are # participants and # districts.

For best performance, you can save the counties in a HashSet that contains O (1), which means O (n) as a whole.

Of course, if the number of counties is small, it does not matter, really.

EDIT . Just noted that your list does not contain identifiers, but complete objects. In order for the above code to work, you also need to change your linq query from select c to select c.Id or something like this (I don't know the name of the field).

Answer 2

 participants = participants .Where(p => counties.Any(c=> c.CountyId == p.CountyOfParticipationId) ) 

Or

 participants.Where(p => p.County.CollaborationId == collaborationId) 

should also work if you set up the relationship correctly

Answer 3

In some situations, this may be better, since you will not need to copy the counties separately if the linq method translates the expression into sql for the values.

 participants = (from p in participants join c in db.CountyCollaborations .Where(cty=>cty.CollaborationId == collaborationId) on p.CountyOfParticipationId equals c.CountyId select p); 

Answer 4

Assuming there is CountyId in every county:

 participants = participants.Where( p => counties.Select(c=> c.CountyId ).Contains( p.CountyOfParticipationId) );