Is Empty structure allowed in C#?

No

yes it may be public struct sts { } static void Main(string[] args) { sts stm; }

yes it may be public struct sts { } static void Main(string[] args) { sts stm; }

No, Its not allowed. A struct can't be null in the same way that an int can't be null, and a float can't be null - they're all value types (and really, a developer should care if they're on the stack rather than on the heap). Nullable types make dealing with SQL a lot easier...