Question

I have the following Linq to Objects query:

var query = this._vaporizerData
    .Where(row => row.Coater == coater)
    .Where(row => row.Distributor == distributor)
    .Where(row => row.PowerTubeA.HasValue);

if (query.Any())
{
    tubeA = query
        .Where(row => row.CoaterTime.Value >= readTime.AddMinutes(-5))
        .Where(row => row.CoaterTime.Value <= readTime)
        .OrderByDescending(row => row.CoaterTime)
        .Select(row => row.PowerTubeA)
        .First()
        .Value;
}

I know that when the query.Any() line is executed, the first Linq query is evaluated. My question is this. Let’s assume the results of the first query is 5 rows of data. When I execute the second query (starting with ‘tubeA = query’), am I correct it will evaluate this against just the five rows returned from the first query?

Thanks very much.

Answer 1

I know that when the query.Any() line is executed, the first Linq query is evaluated.

Well, sort of. It’s evaluated as far as necessary. So if you have a million rows in _vaporizerData but the first of them matches the filter, it’s not going to iterate over the rest of the data.

When I execute the second query (starting with ‘tubeA = query’), am I correct it will evaluate this against just the five rows returned from the first query?

Yes, but it will have to find those rows again, checking the filter against all the rows which didn’t match the first time. So again, if you have a million rows in _vaporizerData but this time only the last row matches the original query, you’ll end up checking against all the 999,999 non-matching rows again.

Also, if anything else changes the contents of _vaporizerData between the call to Any() and the call to First(), those changes will be visible, as it really is going back to the original data source.

It would be better to only execute the query once, e.g.

var minTime = readTime.AddMinutes(-5); // Avoid recalculation
var result = this._vaporizerData
                 .Where(row => row.Coater == coater)
                 .Where(row => row.Distributor == distributor)
                 .Where(row => row.PowerTubeA.HasValue);
                 .Where(row => row.CoaterTime.Value >= minTime)
                 .Where(row => row.CoaterTime.Value <= readTime)
                 .OrderByDescending(row => row.CoaterTime)
                 .Select(row => row.PowerTubeA)
                 .FirstOrDefault();

if (result != null)
{
    tubeA = result.Value;
}

EDIT: Two important points noted in comments:

• Your original query would throw an exception if we the first query matched an item but the second didn’t; the code above would just end up with result as null. That may or may not be an issue for you.
• If you use MoreLINQ you could make this more efficient by using .MaxBy(row => row.CoaterTime) instead of using OrderByDescending and First. IIRC, that will fail on an empty input though, so you should only do this if you’re using your original version with First, not the version above.