UrlResolutionTagHelper doesn't work with TagHelpers that run before and supress output.

[NTaylorMullen]

If a user has a TagHelper and sets its order to int.MinValue, it'll run before the UrlResolutionTagHelper. Now, if that same TagHelper calls output.SuppressOutput() it will then force the TagHelperOutputs tag name to null. The problem this introduces is the UrlResolutionTagHelper relies on the TagName not being null when looking up corresponding attribute values for the tag so it ends up exploding here.

We should check if TagName == null and if so, bail early.

[javiercn]

Would other taghelper types be affected by this same issue? the code that has the error seems applicable in many tag helpers (do we have more cases like this on our codebase)? /cc: @NTaylorMullen

[NTaylorMullen]

Very possible. Would have to give them a closer look or build out a sample to verify. When we wrote them initially we tried to ensure they could react to other TagHelpers in the pipeline; however, that was lower priority. We covered a lot of cases but obviously not everything 😄

[javiercn]

I looked up the implementation of all tag helpers I could find, I didn't see a similar pattern, so I think it's just this one