What does Enum.GetUnderlyingType(Type) do?

Question

I don't understand the purpose of Enum.GetUnderlyingType(Type enumType)

The MSDN documentation doesn't help either:

Returns the underlying type of the specified enumeration.

It seems that this converts a specified type of enum into... something else. o_O

What IS an underlying type? This looks like some internal details of the implementation. Why is this public? Why would I care about the implementation? Browsing the actual implementation doesn't help either, the method just does some checks then calls

[MethodImplAttribute(MethodImplOptions.InternalCall)] 
private static extern Type InternalGetUnderlyingType(Type enumType);

... to which I can't find the source.

Can anybody shed some light on this?

Answer 1

Note that you can specify the underlying type of an enum via

enum Foo : long { One, Two };

And then GetUnderlyingType is going to return long for typeof(Foo).

Note that the underlying type can be any integral type except for char types.

Answer 2

Enums are stored in memory as numbers. By default, int32. That's the underlyting type. You can change that:

public enum z : byte {
  x = 257 // invalid
}