How to avoid getting "possible "null"assignment to a non-nullable entity"

Question

Sorry for this awkward example, but I want to keep it as to the point as possible

Lets say, I have parsed a json-file, which gives me the hierarchy below

var customers = myObj.SubObj?.CustomerList.EmptyIfNull();
var plusCustomers = customers.Where(...) // Resharper warning

myobj cannot be null SubObject can be null sometimes

I have an extension method

IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> enumerable) => enumerable ?? Enumerable.Empty<T>();

So I expect the plusCustomers-line to be safe(empty list instead of null). But I get a warning

possible "null"assignment to a non-nullable entity

If I replace the first line with

var customers = myObj.SubObj.CustomerList.EmptyIfNull();

I get rid of the warning, but that is not correct. How can I convince Resharper that this is ok?...or I'm I mistaken?

Answer 1

You can add brackets:

(myObj.SubObj?.CustomerList).EmptyIfNull();

The effect of the null chaining (?.) is to evaluate the whole chain of member accesses to null, if the LHS operand is null, rather than evaluating just that single member access to null. For example, consider a long chain of member accesses a.b.c.d.e.f. If only a is nullable, you would only need to write ? after a for this to be safe. It doesn't just evaluate a?.b to null if a is null, because in that case you'd have to write ? after every member access for this to be safe, and that's a bit annoying.

So without the brackets, the whole member access is made nullable due to ?., regardless of what you call at the end.

By adding brackets, you break up the chain of member accesses, so that myObj.SubObj?.CustomerList is of a nullable type, and calling EmptyIfNull on that works as expected - returning whatever type EmptyIfNull should return.