static void Main(string[] args)
        {
            using (var db = new DbManager())
            {
                db.SetSpCommand(""); // ok
                db.SetInsertCommand(""); // ok
                db.SetUpdateCommand(""); // ok
                db.Prepare(); // ok
                db.ExecuteNonQuery(); // ok

                db.SetCommand(""); // ERROR:
                    // error CS0584: Internal compiler error: Object reference not set to an instance of an object
                    // error CS0201: Only assignment, call, increment, decrement, and new object expressions can be used as a statement

            }
        }