This one is a quite stupid problem but as it took me 2 hours to figure it out I’ve decided to write it down.
The problem was related to the fact that on a given controller I had a method
public async Task<LookupData> GetAsync()
which I expected based on my WebAPI v1 days to be HttpGet ONLY by convention.
What I have learned is that is not the case in AspNetCore so this method was triggered also by CORS preflight OPTIONS call and that caused all kind of troubles.
Solution to my problem ended very simple – add a [HttpGet] attribute
[HttpGet] public async Task<LookupData> GetAsync()
I somehow don’t like to use attributes when I don’t really have to but in this case lesson learn – annotate your AspNetCore controller methods! 🙂