What kind of dll do you mean? If you mean extension for WED or similar, yes there have been some, but not that many because writing dlls for the editor is sometimes a rough work, I experienced that myself some years ago with A6, it really easier writing dlls for the engine itself rather than the editors. But it's not the fault of the sdk it's just that you have to deal more with severaly things of the winapi especially with messages between processes etc. And nowadays mostly don't do it in the traditional way with winapi but with other things like the .Net framework whis is a lot easier to handle for such purpose.( But only because it hides a lot of things and takes control of things where you lose control lol so it's not a perfect thing
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