I work for an international Content Management Provider and our flagship
product does exactly that.
All we do is have a set path like AppRoot/Plugins and in that path we put
the plugin files.
Each file that wants to be listed as a plugin then needs to use an
interface.
The core then calls up a method on the interface to get the plugin
information and where required execute its entry point.
Because our apps are web apps we also have to add the registration ststement
in the web.config file.
its very simple.
Similar systems have been applied to desktop apps, i wrote something very
much like that some time ago whilst learning some concepts surrounding
interfaces in .NET
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