GetName for enum with duplicate values

Experimentation shows that:

V1 = "value1"

and

V2 = "value1"

However, this isn't guaranteed. The MSDN page on Enum.GetName states:

If multiple enumeration members have the same underlying value, the GetName method guarantees that it will return the name of one of those enumeration members. However, it does not guarantee that it will always return the name of the same enumeration member. As a result, when multiple enumeration members have the same value, your application code should never depend on the method returning a particular member's name.

I disagree with other answers statements

... this isn't guaranteed ...

... you cannot rely on that ...

as well as with msdn statement:

... your application code should never depend on the method returning a particular member's name ...

The story

There was an enum in my software

enum Blabla { A = 0, B = 1, C = 2, D = 3 }

at some point A value changes to AA and later AA changes to AAA. To keep backward compatibility I had to do

enum Blabla { A = 0, AA = 0, AAA = 0, B = 1, C = 2, D = 3 }

This allows to deserialize old enum value (made by older versions of software) as AAA.

Then there was a report which prints Blabla setting value. And at some point every customer using new version start telling me what instead of AAA they see AA value. All of them see AA (and no one report seeing A).

What I did? I simply change the order (until result was AAA)

enum Blabla { AAA = 0, A = 0, AA = 0, ...}

and made a test to ensure what Blabla.AAA will be output as AAA. Problem solved?

The proof

Looking at sources of Enum.ToString() (or Enum.GetName()), it uses GetEnumName(), which calls Array.BinarySearch() for sorted array of values to find an index of value.

The result of binary search is deterministic: providing it with the same parameters will return same result.

So:

• if you don't change enum, then result will be the same.
• it is possible to find result experimentally (or perhaps by understanding how binary search works and how enum will be processed).
• there is no easy rule to determine result (e.g. you can't say "it always return first defined value").
• it's unlikely what enum format will be changed (e.g. order of definition and order of list of values differs) or Enum.ToString() will be changed, it can happens however, so make sure you have tests for cases where you rely on return value.