This is mostly in response to this StackOverflow Question, and partly because I always forget some of the details on using Shadow Copy.

Obviously this would need a lot more error checking etc, but should be a good starting point.

If you are writing a plugin framework for your application, I recommend looking into an existing framework for it such as MEF, rather than rolling your own.

PluginDemo.Common

This contains all your common code for plugins - pretty much what you would distribute as your app's SDK. In this case, we just have one interface called IPlugin, which we use for finding our plugin's types.

TestPluginOne

This is your plugin - note the reference to PluginDemo.Common is CopyLocal = false - we don't need a copy of it as we have our own version in the host.

PluginDemo.Host

This is your main application - note it only has a reference to PluginDemo.Common, not to TestPluginOne. Points of interest here are:

• Scanner:

  • This must inherit from MarshalByRefObject, as it will be marshalled across AppDomains (our application's domain, and the ones we create for our plugins.)
  • You must not polute your main AppDomain with types from the plugins - if a method in Scanner returns a Type that is not available in your AppDomain, you will get an FileNotFoundException, with the name of the assembly which the type came from. This is what Scanner.ShowCrossDomainPolutionExceptions demonstrates.
  • If you are going to have plugins which live for a long time (greater than 5mins, if I remember correctly), then change the Scanner to inherit from the classes in this question on StackOverflow, as otherwise you will run into System.Runtime.Remoting.RemotingException: Object [...] has been disconnected or does not exist at the server. exceptions.

• AssemblyManager:

  • To prevent file locking on our plugins we use Shadow Copying.
  • This will currently leave debris in your temp folder, so don't forget to clear your AppDomain's CachePath after unloading it. You can also set your own CachePath to your own location if you prefer.
  • To allow us to communicate across AppDomains, we create an instance of Scanner using AppDomain.CreateInstanceAndUnwrap. This creates an instance of the Scanner class in the new AppDomain, and returns a proxy to it, which is stored in the current AppDomain (in the _scanner field in this case.) You can see this by setting a break point on the line and inspecting _scanner (it will be a __TransparentProxy type.)

• Program:

  • Uses the AssemblyManager to load TestPluginOne.
  • Calls a method on a type in TestPluginOne.
  • Deletes the TestPluginOne loaded from.
  • Calls the same method in TestPluginOne (showing that we are not using the local copy of the dll).
  • Calls the Cross Domain Pollution test method.