Issue with select() and IsInInstance()

[SDPhantom]

Correct, C code is completely out of the scope of what addons have access to. This barrier is by design to allow UI customization, yet keep people from hacking the game itself from within. This is what is known as a "sandbox". You can do whatever you want within the Lua environment, but you can't affect or respond from anything outside of it, including C code that runs behind the WoW API.