When a client sends in multiple values in Accept header doing a check on that header fails the Contains method.

Looking at the Accept header it is a StringValues type which implements the many ICollection,IList,IEnumerable etc interfaces and the value of the Accept header is 1 entry with a string separated by comma. Therefore a IList.Contains will fail because there isn't an entry that matches application/json when the entry is application/json, text/plain, */*

I would have expected 3 entries in the result of calling context.Request.Headers["Accept"] which would have made the check pass. For now I will have to use context.Request.Headers["Accept"].Any(h => h.Contains("application/json")) or context.Request.Headers["Accept"].ToString().Contains("application/json"))

Attached is a screenshot of my debug window which shows what I saw.