So I'm writing a Web Api .Net service to call from some mobile apps. Before you ask, I haven't used .Net Core since it requires all the support libraries are portable and that is not a 5 minute job!
Anyway, it basically works but I found a couple of funnies that have been reported elsewhere but they are not things that are obviously broken - thank goodness for Unit Tests!
1) I have an attribute that validates the model required by the API action and then sets BadRequest if the model doesn't validate - this saves calling if (ModelState.IsValid) everywhere. It didn't seem to work, IsValid was true when I called an action with no parameters. The reason? If the model is null, it passes validation! Terrible but true. I had to add an additional line of code to ensure the model was null before checking whether it was valid.
2) The RegularExpressionAttribute does not validate empty strings according to the regex. It would be nice if it was a property of the attribute but it isn't, it just doesn't. Again, I had to subclass RegularExpressionAttribute, override IsValid to ensure the value is not empty and then call the base class IsValid. I then subclassed this into my specific Attributes so that they all work as expected.