Question

Often I want to check if a provided value matches one in a list (e.g. when validating):

if (!acceptedValues.Any(v => v == someValue))
{
    // exception logic
}

Recently, I’ve noticed ReSharper asking me to simplify these queries to:

if (acceptedValues.All(v => v != someValue))
{
    // exception logic
}

Obviously, this is logically identical, perhaps slightly more readable (if you’ve done a lot of mathematics), my question is: does this result in a performance hit?

It feels like it should (i.e. .Any() sounds like it short-circuits, whereas .All() sounds like it does not), but I have nothing to substantiate this. Does anyone have deeper knowledge as to whether the queries will resolve the same, or whether ReSharper is leading me astray?

Answer 1

Implementation of All according to ILSpy (as in I actually went and looked, rather than the “well, that method works a bit like …” I might do if we were discussing the theory rather than the impact).

public static bool All<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    foreach (TSource current in source)
    {
        if (!predicate(current))
        {
            return false;
        }
    }
    return true;
}

Implementation of Any according to ILSpy:

public static bool Any<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    foreach (TSource current in source)
    {
        if (predicate(current))
        {
            return true;
        }
    }
    return false;
}

Of course, there could be some subtle difference in the IL produced. But no, no there isn’t. The IL is pretty much the same but for the obvious inversion of returning true on predicate match versus returning false on predicate mismatch.

This is linq-for-objects only of course. It’s possible that some other linq provider treats one much better than the other, but then if that was the case, it’s pretty much random which one got the more optimal implementation.

It would seem that the rule comes down solely to someone feeling that if(determineSomethingTrue) is simpler and more readable than if(!determineSomethingFalse). And in fairness, I think they’ve a bit of a point in that I often find if(!someTest) confusing* when there’s an alternative test of equal verbosity and complexity that would return true for the condition we want to act upon. Yet really, I personally find nothing to favour one over the other of the two alternatives you give, and would perhaps lean very slightly toward the former if the predicate were more complicated.

*Not confusing as in I don’t understand, but confusing as in I worry that there’s some subtle reason for the decision that I don’t understand, and it takes a few mental skips to realise that “no, they just decided to do it that way, wait what was I looking at this bit of code for again?…”