I\'m building a project. It\'s an application that users can add their extensions (DLL files) to. To manage an extension, I need to get its list of classes, functions, etc.
For a particular assembly, you can use Assembly.GetTypes to get the types, then for each type call Type.GetMethods(), Type.GetProperties() etc, or just Type.GetMembers().
However, for plugin functionality it's usually a good idea to have a common interface which the plugins have to implement - that reduces the amount of reflection you need to use. Use Type.IsAssignableFrom() to check whether a type is compatible with a particular interface.
You might also want to look at the Managed Extensibility Framework which can make implementing an extension system easier.
If you're talking about building an extensible application in C# that loads other assemblies and uses their contents for extending the application, then see Stack Overflow question System with plugins in C#.