Question

I think my point is clear, under any condition, could this throw null-reference exception? i.e. objecting that e.Result is null while trying to access its Count property.

if (e.Result == null || e.Result.Count == 0)
    return;

EDIT: To gain the most of this question (for later-coming askers), how will we prevent that exception from happening, can we use lock statement? how?

Answer 1

That code can throw a NullReferenceException only if the method is not thread safe. Meaning, if another thread decides to assign null to e.Result at coincidentally the exact moment after your e.Result == null check and right before your e.Result.Count == 0 check.