Why can't I derive from System.Enum abstract class?

Question

MSDN defines System.Enum as an abstract class:

[SerializableAttribute]
[ComVisibleAttribute(true)]
public abstract class Enum : ValueType, IComparable, IFormattable, IConvertible

And also MSDN make such statement about abstract class:

An abstract class cannot be instantiated. The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share.

But I simply cannot derive from System.Enum. According to Jeffery Richter in his book "CLR via C#", it is C# compiler that forbid that derivation.

And I checked System.Enum's members, most of them are static, which is reasonable for it cannot be instantiated and static methods don't need an object instance to invoke. But there are also some instance methods, such a GetHashCode() and GetTypeCode().

So, my first question is, if System.Enum cannot be instantiated or derived, how could these instance methods be invoked? Isn't this some kind of a paradox?

I know I could use the following approach to invoke these instance methods, but why? Is there some kind of System.Enum or derived type object instance created? When? and by whom?

public enum Days:byte { Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday };
Days d = Days.Friday;
d.GetTypeCode();
d.GetHashCode();

Answer 1

You can derive from Enum, e.g.

public enum MyEnum
{
    // ...
}

Will be compiled down to something like:

.class public auto ansi sealed MyEnum
    extends System.Enum
{
    // ...
}

The restriction the C# compiler places is that it won't let you write the derivation manually, but instead requires you to use the enum keyword when declaring the type.

As to why this is... my guess is because enumerations are a bit weird in the CLR. For example, the ValueType and Enum base types are both themselves reference types, not value types. There's a whole bunch of magic going on under the covers, and if you force the use of a keyword and prevent derivation from the magic base class, then you are keeping your intent separate from the magic.