How is it that an enum derives from System.Enum and is an integer at the same time?

Question

Edit: Comments at bottom. Also, this.

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Here\'s what\'s kind of confusing me. My understanding is that if I have an enum like this...

Answer 1

Why not... it is perfectly valid, for example, for a structure to hold an int internally, and be convertible to int with an explicit cast operator... lets simulate an Enum:

interface IEnum { }

struct MyEnumS : IEnum
{
    private int inner;

    public static explicit operator int(MyEnumS val)
    {
        return val.inner;
    }

    public static explicit operator MyEnumS(int val)
    {
        MyEnumS result;
        result.inner = val;
        return result;
    }

    public static readonly MyEnumS EnumItem1 = (MyEnumS)0;
    public static readonly MyEnumS EnumItem2 = (MyEnumS)2;
    public static readonly MyEnumS EnumItem3 = (MyEnumS)10;

    public override string ToString()
    {
        return inner == 0 ? "EnumItem1" :
            inner == 2 ? "EnumItem2" :
            inner == 10 ? "EnumItem3" :
            inner.ToString();
    }
}

This struct can be used quite the same way a struct can... of course, if you try to reflect the type, and call IsEnum property it will return false.

Let's look at some usage comparison, with the equivalent enum:

enum MyEnum
{
    EnumItem1 = 0,
    EnumItem2 = 2,
    EnumItem3 = 10,
}

Comparing usages:

Struct version:

var val = MyEnum.EnumItem1;
val = (MyEnum)50;
val = 0;
object obj = val;
bool isE = obj is MyEnum;
Enum en = val;

Enum version:

var valS = MyEnumS.EnumItem1;
valS = (MyEnumS)50;
//valS = 0; // cannot simulate this
object objS = valS;
bool isS = objS is MyEnumS;
IEnum enS = valS;

Some operations cannot be simulated, but this all shows what I intended to say... Enums are special, yes... how much special? not that much! =)

Answer 2

A Enum's underlying type is the type used to store the value of the constants. In your example, even though you haven't explicitly defined the values, C# does this:

enum Animal : int
{
    Dog = 0,
    Cat = 1
}

Internally, Animal is made up of two constants with the integer values 0 and 1. That's why you can explicitly cast an integer to an Animal and an Animal to an integer. If you pass Animal.Dog to a parameter that accepts an Animal, what you are really doing is passing the 32bit integer value of Animal.Dog (in this case, 0). If you give Animal a new underlying type, then the values are stored as that type.