Doomsday Engine

Thats not possible. When a game is unloaded the entire definition database is reset before then parsing only those definitions which should be present in Ring Zero.

Can you explain the problem you have observed and provide step by step instructions on how to reproduce it.

I'm using this mod with Dday, that comes with an external Dehacked patch.
http://www.doomworld.com/idgames/index.php?file=levels/doom2/Ports/m-o/marspr9c.zip

1. I launch Doom2 with the pwad and deh, through Snowberry.
2. I then unload the game (console command or taskbar).

If I then try to load any game (even Heretic or HeXen) through Ring Zero, the mods Dehacked patch is still being applied, with a variety of negative results on each games mobjs (for instance a crash at the start of E1M1 in Heretic's case).

It sounds like the game session file list is not being cleared upon unloading the game, leading to the DEHACKED patch being loaded again when the current game is changed. I'll investigate...

I would like to add that this issue also affects ded's.

One can easily see such, by loading a ded that references a non-existing mobj state (the alert is repeated every time one returns to Ring Zero or loads a game).

To fix the underlying issue the Right Way, requires implementing a new mechanism for describing game sessions; including the ordered list of add-ons, values for what were previously the games' command line options, and the session ID for multiplayer.

While it would be straightforward to add a one-line check to only load add-ons specified on the command line, when changing game if -game is specified and valid - this wouldn't address the issue of repeat application of command line options intended for the game. As such, doing so would introduce unintuitive behavior.

Consequently, although incorrect and unexpected, the current behavior is good enough until such time as a proper mechanism for game session handling (and persistence thereof) is implemented.