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...

No

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