Define and use a enum

Be default, enum start element from 0. And enum's default value is 0; So if just declaring a enum member (local variable must be assigned a value) without assigning value, its value will be the first element.

                            public enum Day{
                                Sun,
                                Mon,
                                Tue,
                                Wed,
                                Thu,
                                Fri,
                                Sat,
                            }
                            Day day = Day.Sun;

                            Day day2; // because enum is value type, it must have a default value, which is 0;
                            Console.WriteLine(day2);
                        

Even though the underlying data type is int, you cannot directly assign a integer to it.

Check it has a valid value.

                            public enum State{
                                GA = 1, MN, CO
                            }
                            State s = default(State); // 0
                            // but 0 is not a valid member.
                            Console.WriteLine(Enum.IsDefined(typeof(State), s)); // false
                        

string -- Enum

                            public enum Rating{
                                G, PG, PG13, R, NC17 
                            }
                            Console.WriteLine("Hello World!");

                            Rating r = Rating.PG13;
                            Console.WriteLine(r); // PG13;
                            Console.WriteLine(r.ToString()); // PG13;
                            string pg13 = "PG-13";
                            try{
                                r = (Rating)Enum.Parse(typeof(Rating), pg13);
                            }catch(Exception e){
                                Console.WriteLine(e.Message); // Requested value 'PG-13' was not found.
                            }

                            bool result = Enum.TryParse<Rating>(pg13, out r);
                            Console.WriteLine($"{result}, {r}"); // false, G      default value.
                        

int -- enum

                            int number = (int)Rating.PG13;
                            Rating r = (Rating)1;
                            Console.WriteLine(r); // PG
                            r = (Rating)100;
                            Console.WriteLine(r); // 100
                        

int --> string

                            // public static string GetName (Type enumType, object value);
                            Console.WriteLine(Enum.GetName(typeof(Rating), 1)); // PG
                            string s = Enum.GetName(typeof(Rating), 100); // return null