Does adding enum values break binary compatibility?

Question

Imagine this enum in a DLL.

public enum Colors
{
    Red,
    Green
}

Does adding enum values break binary compatibility? If I were to change it, would existing EXEs break?

public enum Colors
{
    Red,
    Green,
    Blue
}

I saw this answer, but it seemed to address the case of inserting a value. If I add values to the end only, is that OK?

Answer 1

No, this doesn't break binary compatibility (in as much as: the assembly will still load etc), because enums are basically integer literal constants. Inserting values in the middle is obviously a really dangerous idea, but you've already excluded that.

However, it can cause a number of other issues that you need to guard against:

• some code (switch statements in particular) may not anticipate the new values; technically this was an issue before too, since enums are not value-checked (enum variables can contain undefined values)
• anything that queries the available enums is going to get different results
  • in particular, serialization and deserialization may fail unexpectedly if there is data using enums that aren't yet expected by the particular client