How to avoid Linq chaining to return null?

Question

I have a problem with code contracts and linq. I managed to narrow the issue to the following code sample. And now I am stuck.

public void SomeMethod()
{
    var list = new List<Question>();

    if (list.Take(5) == null) { }
    // resharper hints that condition can never be true

    if (list.ForPerson(12) == null) { }
    // resharper does not hint that condition can never be true
}

public static IQueryable<Question> ForPerson(this IQueryable<Question> source, int personId)
{
    if(source == null) throw new ArgumentNullException();

    return from q in source
           where q.PersonId == personId
           select q;
}

What is wrong with my linq chain? Why doesn't resharper 'complain' when analyzing the ForPerson call?

EDIT: return type for ForPerson method changed from string to IQueryable, which I meant. (my bad)

Answer 1

Reshaper is correct that the result of a Take or Skip is never null - if there are no items the result is an IEnumerable<Question> which has no elements. I think to do what you want you should check Any.

var query = list.Take(5);
if (!query.Any())
{
    // Code here executes only if there were no items in the list.
}

But how does this warning work? Resharper cannot know that the method never returns null from only looking at the method definition, and I assume that it does not reverse engineer the method body to determine that it never returns null. I assume therefore that it has been specially hard-coded with a rule specifying that the .NET methods Skip and Take do not return null.

When you write your own custom methods Reflector can make assumptions about your method behaviour from the interface, but your interface allows it to return null. Therefore it issues no warnings. If it analyzed the method body then it could see that null is impossible and would be able to issue a warning. But analyzing code to determine its possible behaviour is an incredibly difficult task and I doubt that Red Gate are willing to spend the money on solving this problem when they could add more useful features elsewhere with a much lower development cost.

To determine whether a boolean expression can ever return true is called the Boolean satisfiability problem and is an NP-hard problem.

You want Resharper to determine whether general method bodies can ever return null. This is a generalization of the above mentioned NP-hard problem. It's unlikely any tool will ever be able to do this correctly in 100% of cases.

Answer 2

if(source == null) throw new ArgumentNullException(); 

That's not the code contract way, do you instead mean:

Contract.Require(source != null);