Question

I have the following function that returns results from the database based on LINQ Expressions:

IQueryable<TEntity> FindAll<TEntity>(Expression<Func<TEntity, bool>> expression)

When I try pulling data from the function while using the .Any function from a list I get a null reference exception.

However when I pull the data without that specific condition and use the same .Any function in a for each loop everything works correctly.

Here is the call trying to use the .Any function which does not work:

var ppcReports = repository.FindAll<PPCReport>(
    x => x.ClientId == clientId && 
    (campaigns.Any(c=> c.Id == x.CampaignId))
).ToList();

And the way it does work properly:

var ppcReports = repository.FindAll<PPCReport>(
    x => x.ClientId == clientId).ToList();

foreach (var item in ppcReports)
{
    if (campaigns.Any(c => c.Id == item.CampaignId))
    {
        // do something
    }
}

I was wondering why was this happening, am I doing something wrong is it just not possible to filter the results before the query finished?

By calling .ToList() before filtering the results it does work, so I suppose I cannot do such an operation on an IQueryable<T> implementation?

var ppcReports = repository.
    FindAll<PPCReport>(x => x.ClientId == clientId).
    ToList().
    Where(w => campaigns.Any(c => c.Id == w.CampaignId)).
    ToList();

Answer 1

Like those who commented, I’m surprised that you got a NullReferenceException rather than a complaint about not being able to compile that statement to SQL. However, the following code should let you do this in 1 query (and will do all filtering in SQL):

var campaignIds = (campaigns ?? Enumerable.Empty<Campaign>())
    .Select(c => c.Id);
var ppcReports = repository
    .FindAll<PPCReport>(pr => pr.ClientId == clientId
        && campaignIds.Contains(pr.CampaignId))
    .ToList();

This should work in both EF and Linq-to-SQL.