EF Core - FirstOrDefaultAsync throws Null Reference Exception

Question

I am trying to test ASP.NET Core Web API controllers using XUnit. I am using the DBContext directly in the controller. In production, I am using SqlServer and for testing, I am using InMemory provider. When I am testing some code which is using the EF Core FirstOrDefaultAsync method, the application is throwing null reference exception. The value I am looking for doesn't exist in the DB. As per my understanding, it should return NULL, it should not throw an exception.

I tried something like where(x => x.Id = id).FirstOrDefaultAsync(), it is also throwing the same null reference exception.

When I tried something like

var exist = await list.AnyAsync(x => x.Id == id);
if(!exist)
{
return NotFound();
}
var user = await list.FirstAsync(x => x.Id == id);
return user;

It works. Both dbcontext and users property are got values, those are not null.

Please help.

what is `list` here? `IEnumerable` right? how you got this list? from context? — Deepak Sharma, Jul 25 '19 at 04:00
@mjwills It is 10. It is an InMemoryDatabase context, which I am using for testing. So I am hard coding the values to 10 — Anuraj, Jul 25 '19 at 04:35
@mjwills Here is the gist - https://gist.github.com/anuraj/6cb319eb68ee56d9bd4f29b80f8d2f9f — Anuraj, Jul 25 '19 at 04:55
agree with @mjwills suggestion, need some minimal reproducible example, with such statement nd single line output, we can't identity exact scenario — Deepak Sharma, Jul 25 '19 at 04:55
Which part is null here exactly? The error should say. The context isn’t null itself? — Sami Kuhmonen, Jul 25 '19 at 05:00
@SamiKuhmonen I am not able to identify which is null. Both the objects from my code got values, looks like it is thrown from framework. — Anuraj, Jul 25 '19 at 05:05

score 5 · Answer 1 · answered Jul 25 '19 at 06:16

5

The exception is not caused by your code. I was able to reproduce it with the latest at this time EF Core 3.0 Preview 7. It happens with the in-memory provider - FirstOrDefault[Async] and SingleOrDefault[Async] methods. Does not happen with First[Async] or Single[Async].

Anyway, the main problem here is that you are using a preview (beta) software, which is expected to have issues.

Simply switch to the latest stable EF Core 2 version and wait for 3.0 release before trying to use it in production code.

answered Jul 25 '19 at 06:16

Ivan Stoev

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2

I am using EF Core v2.1 and got the very same issue as the OP – M. Ruiz Oct 11 '19 at 21:21
@M.Ruiz I doubt it is the same - this was really 3.0 preview related and wan't reproducing with the latest stable 2.x. Might be something similar though. Consider asking own question with details about your issue. – Ivan Stoev Oct 14 '19 at 15:31
2

Excuse me but I'm not asking any question, I'm stating that I have the same issue using v2.1 which is LTS, or at least that's what I thought. Cheers mate. – M. Ruiz Oct 14 '19 at 16:44
1

I'm having the same issue as well. Using v2.2. – Laurie Dickinson Oct 24 '19 at 20:31
Same issue here, I did ask a question about it here on stackoverflow a few days ago, didn't get any answers though, however I did add some code to reproduce it. >https://stackoverflow.com/questions/59500303/why-is-test-failing-when-using-firstordefaultasync-in-method – Noob Dec 30 '19 at 15:30
@Noob You are mixing EF Core specific extension method (supposed to work only with EF Core provided `IQueryable<>` implementations) with mocked implementation. I have no idea what that mocking library is doing and what EF Core expects. If the standard sync methods (`First`, `FirstOrDefault` etc.) work and Async versions fail, then I guess the problem is in mocking library implementation. – Ivan Stoev Dec 30 '19 at 15:47
@IvanStoev ow, I'll try asking the maker of the library then, maybe he has some examples how to do it properly using firstordefaultasync. Anyway thanks. – Noob Dec 30 '19 at 15:57

EF Core - FirstOrDefaultAsync throws Null Reference Exception

1 Answers1