Use this string when creating your AddIn and using the AddInVersionAttribute on your AddIn assembly. This will ensure that your AddIn captures the API version it was written against, and will prevent it from loading if the API version changes due to a breaking change.

A public const field is compiled into the assembly that references it, so it is captured as a snapshot of the moment of compilation. Thus if the AddIn API version changes, AddIn assemblies will need to be rebuilt to pick up the new version number, and any breaking API changes will be discovered at compile time.