C#: Should the default value of an enum be None or Unknown?

Question

Say you have an enum that represents an error code. There will be several codes, each with their own underlying int value; however, the enum value that gets the default 0 value seems like it should be carefully considered.

In the case of an error code enum, there are two special values that I can think of: None (for cases when there is no error) and Unknown (for cases when no existing error code is appropriate, or perhaps even when error state cannot be detected).

One of these values seems like it should get 0, where the other will probably get something else like -1. Is it more appropriate to set the None value to 0, or the Unknown value to 0?

public enum ErrorCode
{
    None = -1,
    Unknown = 0,
    InsufficientPermissions,
    ConnectivityError,
    ...
}

public enum ErrorCode
{
    Unknown = -1,
    None = 0,
    InsufficientPermissions,
    ConnectivityError,
    ...
}

My instinct tells me that the default should be Unknown, but I'm curious if anyone has done it differently.

Answer 1

Since you're tagging your question with best-practices: don't use error codes when you can use exceptions.

Answer 2

In my opinion both Unknown or None mean the same thing in the context of the ErrorCode enumeration. My reasoning is that if I'm checking an error code than it's because I already have an error.

I'm also of the opinion that an error code enumeration is only useful in a custom exception or as custom data of an existing exception type and in both scenarios you will always have an error.