c# - Should I use two "where" clauses or "&&" in my LINQ query?

Question

When writing a LINQ query with multiple "and" conditions, should I write a single where clause containing && or multiple where clauses, one for each conditon?

static void Main(string[] args)
{
    var ints = new List<int>(Enumerable.Range(-10, 20));

    var positiveEvensA = from i in ints
                         where (i > 0) && ((i % 2) == 0)
                         select i;

    var positiveEvensB = from i in ints
                         where i > 0
                         where (i % 2) == 0
                         select i;

    System.Diagnostics.Debug.Assert(positiveEvensA.Count() == 
                                         positiveEvensB.Count());
}

Is there any difference other than personal preference or coding style (long lines, readability, etc.) between positiveEvensA and positiveEvensB?

One possible difference that comes to mind is that different LINQ providers may be able to better cope with multiple wheres rather than a more complex expression; is this true?

Answer 1

I personally would always go with the && vs. two where clauses whenever it doesn't make the statement unintelligible.

In your case, it probably won't be noticeble at all, but having 2 where clauses definitely will have a performance impact if you have a large collection, and if you use all of the results from this query. For example, if you call .Count() on the results, or iterate through the entire list, the first where clause will run, creating a new IEnumerable<T> that will be completely enumerated again, with a second delegate.

Chaining the 2 clauses together causes the query to form a single delegate that gets run as the collection is enumerated. This results in one enumeration through the collection and one call to the delegate each time a result is returned.

If you split them, things change. As your first where clause enumerates through the original collection, the second where clause enumerates its results. This causes, potentially (worst case), 2 full enumerations through your collection and 2 delegates called per member, which could mean this statement (theoretically) could take 2x the runtime speed.

If you do decide to use 2 where clauses, placing the more restrictive clause first will help quite a bit, since the second where clause is only run on the elements that pass the first one.

Now, in your case, this won't matter. On a large collection, it could. As a general rule of thumb, I go for:

1. Readability and maintainability

2. Performance

In this case, I think both options are equally maintainable, so I'd go for the more performant option.