How do I declare a nested enum?

Question

I want to declare a nested enum like:

\\pseudocode public enum Animal {   dog = 0,   cat = 1 }  private enum dog {    bulldog = 0,    greyhound = 1,    husky = 3 }  private enum cat {    persian = 0,    siamese = 1,    burmese = 2 }  Animal patient1 = Animal.dog.husky; 

Can it be done?

Answer 1

I was looking for something similar as a way to create lightweight, hierarchical channel ID's for a logging system. I'm not quite sure this was worth the effort, but I had fun putting it together, and I learned something new about operator overloading and lizards in the process.

I've built a mechanism that supports this notation:

public static class Animal {     public static readonly ID dog = 1;     public static class dogs     {         public static readonly ID bulldog = dog[0];         public static readonly ID greyhound = dog[1];         public static readonly ID husky = dog[3];     }      public static readonly ID cat = 2;     public static class cats     {         public static readonly ID persian = cat[0];         public static readonly ID siamese = cat[1];         public static readonly ID burmese = cat[2];     }      public static readonly ID reptile = 3;     public static class reptiles     {         public static readonly ID snake = reptile[0];         public static class snakes         {             public static readonly ID adder = snake[0];             public static readonly ID boa = snake[1];             public static readonly ID cobra = snake[2];         }          public static readonly ID lizard = reptile[1];         public static class lizards         {             public static readonly ID gecko = lizard[0];             public static readonly ID komodo = lizard[1];             public static readonly ID iguana = lizard[2];             public static readonly ID chameleon = lizard[3];         }     } } 

And which you can use like so:

void Animalize() {     ID rover = Animal.dogs.bulldog;     ID rhoda = Animal.dogs.greyhound;     ID rafter = Animal.dogs.greyhound;      ID felix = Animal.cats.persian;     ID zorro = Animal.cats.burmese;      ID rango = Animal.reptiles.lizards.chameleon;      if (rover.isa(Animal.dog))         Console.WriteLine("rover is a dog");     else         Console.WriteLine("rover is not a dog?!");      if (rover == rhoda)         Console.WriteLine("rover and rhoda are the same");      if (rover.super == rhoda.super)         Console.WriteLine("rover and rhoda are related");      if (rhoda == rafter)         Console.WriteLine("rhoda and rafter are the same");      if (felix.isa(zorro))         Console.WriteLine("er, wut?");      if (rango.isa(Animal.reptile))         Console.WriteLine("rango is a reptile");      Console.WriteLine("rango is an {0}", rango.ToString<Animal>()); } 

That code compiles and produces the following output:

rover is a dog rover and rhoda are related rhoda and rafter are the same rango is a reptile rango is an Animal.reptiles.lizards.chameleon 

Here's the ID struct that makes it work:

public struct ID {     public static ID none;      public ID this[int childID]     {         get { return new ID((mID << 8) | (uint)childID); }     }      public ID super     {         get { return new ID(mID >> 8); }     }      public bool isa(ID super)     {         return (this != none) && ((this.super == super) || this.super.isa(super));     }      public static implicit operator ID(int id)     {         if (id == 0)         {             throw new System.InvalidCastException("top level id cannot be 0");         }         return new ID((uint)id);     }      public static bool operator ==(ID a, ID b)     {         return a.mID == b.mID;     }      public static bool operator !=(ID a, ID b)     {         return a.mID != b.mID;     }      public override bool Equals(object obj)     {         if (obj is ID)             return ((ID)obj).mID == mID;         else             return false;     }      public override int GetHashCode()     {         return (int)mID;     }      private ID(uint id)     {         mID = id;     }      private readonly uint mID; } 

This makes use of:

• a 32-bit uint as the underlying type
• multiple small numbers stuffed into an integer with bit shifts (you get maximum four levels of nested ID's with 256 entries at each level -- you could convert to ulong for more levels or more bits per level)
• ID 0 as the special root of all ID's (possibly ID.none should be called ID.root, and any id.isa(ID.root) should be true)
• implicit type conversion to convert an int into an ID
• an indexer to chain ID's together
• overloaded equality operators to support comparisons

Up to now everything's pretty efficient, but I had to resort to reflection and recursion for ToString, so I cordoned it off in an extension method, as follows:

using System; using System.Reflection;  public static class IDExtensions {     public static string ToString<T>(this ID id)     {         return ToString(id, typeof(T));     }      public static string ToString(this ID id, Type type)     {         foreach (var field in type.GetFields(BindingFlags.GetField | BindingFlags.Public | BindingFlags.Static))         {             if ((field.FieldType == typeof(ID)) && id.Equals(field.GetValue(null)))             {                 return string.Format("{0}.{1}", type.ToString().Replace('+', '.'), field.Name);             }         }          foreach (var nestedType in type.GetNestedTypes())         {             string asNestedType = ToString(id, nestedType);             if (asNestedType != null)             {                 return asNestedType;             }         }          return null;     } } 

Note that for this to work Animal could no longer be a static class, because static classes can't be used as type parameters, so I made it sealed with a private constructor instead:

public /*static*/ sealed class Animal {     // Or else: error CS0718: 'Animal': static types cannot be used as type arguments     private Animal()     {     }     .... 

Phew! Thanks for reading. :-)