Question

Trying to write dynamic queries using the LINQ provider for NHibernate, but I am having issues. My understanding was that LINQ queries were deferred until called, (i.e. with ToList()), so I have the following code:

string[] filteredIds = new[] { "someIdNotInUse"};

var result = _products
    .GetAll()
    .Skip(0)
    .Take(10);

if (filteredIds != null) 
{
    result.Where(x => x.Child1.Child2.Any(z => filteredIds.Contains(z.Child3.Id)));
}

var r = result.ToList();

The Where filter in the conditional block is not applied; when I run .ToList, I get records where I expect none. However, if I remove the where filter and append it directly to the _products call, it works as expected. Am I misunderstanding how the LINQ provider works? How is creating a query like this possible, without rewriting the query for every possible filter condition and combination?

Answer 1

Methods in LINQ don’t affect the object they’re called on – they return a new object representing the result of the call. So you want:

if (filteredIds != null) 
{
    result = result.Where(...);
}

(Think of it as being a bit like calling Replace or Trim on a string – the string is immutable, so it’s only the return value which matters.)