Question

I have a linq subquery as follows:

        for (int i = 0; i < parentAttributes.Length; i++)
        {
            Guid parent = parentAttributes[i];
            var subQuery = from sc in db.tSearchCluendexes
                           join a in db.tAttributes on sc.AttributeGUID equals a.GUID
                           join pc in db.tPeopleCluendexes on a.GUID equals pc.AttributeGUID
                           where a.RelatedGUID == parent && userId == pc.CPSGUID                             
                           select sc.CPSGUID;

            query = query.Where(x => subQuery.Contains(x.Id));
        }

The basic idea is that I filter a list of data (as represented by the query variable) based on the results of a subquery executed an indeterminable number of times.

However I now have an issue whereby I only want to execute the where on the query if the subquery actually contains data and I don’t want to execute the subquery within each iteration of the loop (for example by using a Count() or a ToList()) as this will hit the remote resource. Something like below if we take it that a method HasData() actually returns true or false:

        for (int i = 0; i < parentAttributes.Length; i++)
        {
            Guid parent = parentAttributes[i];
            var subQuery = from sc in db.tSearchCluendexes
                           join a in db.tAttributes on sc.AttributeGUID equals a.GUID
                           join pc in db.tPeopleCluendexes on a.GUID equals pc.AttributeGUID
                           where a.RelatedGUID == parent && userId == pc.CPSGUID                             
                           select sc.CPSGUID;

            if (subQuery.HasData())                
               query = query.Where(x => subQuery.Contains(x.Id));
        }

Suggestions are welcome.

Answer 1

Your suggestions of using Count and ToList are not the fastest way to see if a query has any results. You should use the Any method:

if (subQuery.Any())
{            
   query = query.Where(x => subQuery.Contains(x.Id));
}

Note that this will still be evaluated immediately. If you want to delay execution, you should put it inside the lambda function.

query = query.Where(x => !subQuery.Any() || subQuery.Contains(x.Id));