linq null reference error with orderby but firstordefault works

Question

I have the following code:

IEnumerable<VariantChild> children = src.Variants
    .Where(v => !v.ExcludeFromWebsite && !v.ExcludeFromEcommerce)
    .SelectMany(v => v.Children)
    .Where(c => c.Price > 0 && !c.ExcludeFromWebsite && !c.ExcludeFromEcommerce);

if (children.Any())
{
    VariantChild child  = children.OrderBy(c => c.Price).FirstOrDefault();  // this line is throwing a null reference error

    if (child != null)
    {
        dest.Price = child.Price;
    }
}

But it is throwing a null refence exception on the line highlighted. If I remove the orderby , the code works so I thought it may be to do with the price being null - but it can't be as it is a double.

Having stepped through the code, the .Any() returns true, but if I change that to .Count() , I get the null exception

If I step through and check the children object - I can see the results view is null so shouldn't .Any() return false (although why does FirstOrDefult() return a variant child?

Anyone any ideas what is causing the null exception?

Here is a screenshot of the code as I stepped into it:

Answer 1

Keep in mind that linq expressions are not evaluated when created but only when necessary (ex: ToList , or in your case, OrderBy then FirstOrDefault . See deferred execution .). So the null reference exception can be in the evaluation of children .

Supposing that ExcludeFromWebsite and ExcludeFromEcommerce are booleans and can't be null and Price is a double and can't be null. Only remain Children witch is an IEnumerable (because used in SelectMany ) so it can be null.

Furthermore, in your screen shot, the stack trace refer to the SelectManyIterator .

You should check for Children to not be null :

IEnumerable<VariantChild> children = src.Variants
  .Where(v => !v.ExcludeFromWebsite && !v.ExcludeFromEcommerce)
  .SelectMany(v => v.Children ?? Enumerable.Empty<T>()) // T is the type of element in Children
  .Where(c => c.Price > 0 && !c.ExcludeFromWebsite && !c.ExcludeFromEcommerce);