Are you registering IHttpContextAccessor correctly?
20180918 - Updated the link to HttpServiceCollectionExtensions.cs as the original link no longer worked. Thanks Gareth! After starting to write a service in aspnetcore I needed to access the current HttpContext to get access to the current logged in user. Injecting HttpContext use to be very bad in previous versions of asp.net so I investigated how best to get access to items on it in the new world. This is where I discovered the IHttpContextAccessor interface and a whole new world opened up for me....