I hadn't used async / await in C# at the time so I couldn't answer their question that well. They told me it was ok that I hadn't used it but pressed me for an answer anyway - which is quite odd. I told them my thoughts on best practices for using threads in C# which is as close as I could get. They ignored that completely and launched into a spiel about how async / await works in C# - thanks for the primer guys, but is an interview really the place?