The essential idea to remember is that an interface never contains any implementation. The following restrictions and imitations are natural consequences of this:
You're not allowed any fields in an interface, not even static ones. A field is an implementation of an object attribute.
You're not allowed any constructors in an interface. A constructor contains the statements used to initialize the fields in an object, and an interface does not contain any fields!
You're not allowed a destructor in an interface. A destructor contains the statements used to destroy an object instance.
You cannot supply an access modifier. All methods in an interface are implicitly public.
You cannot nest any types (enums, structs, classes, interfaces, or delegates) inside an interface.