Question

I am building a plugin-type system with each plugin represented as a DLL. I would like to be able to reload them without stopping the main application. This means that they must be loaded at runtime, without pre-built links between them (do a filesearch for dlls and load them). I have this set up using Assembly.LoadFile(filename), however, when I try to use File.Copy to replace the DLL it throws an exception, saying something akin to ‘file in use’. I have tried using AppDomain, loading all plugins through this secondary domain, and unloading it before reloading, but this throws the same exception.

My current code:

        if (pluginAppDomain != null)
            AppDomain.Unload(pluginAppDomain);
        foreach (string s in Directory.GetFiles(path_to_new_DLLs))
        {
            string name = s.Substring(s.LastIndexOf('\\') + 1);
            Console.WriteLine("Copying " + name);
            File.Copy(s, Path.Combine(current_directory, name), true); // Throws exception here
        }
        AppDomainSetup setup = new AppDomainSetup();
        setup.ApplicationBase = Environment.CurrentDirectory;
        setup.ShadowCopyFiles = "true"; 
        // I think this is where the problem is, maybe I'm forgetting to set something
        pluginAppDomain = AppDomain.CreateDomain("KhybotPlugin", null, setup);
        foreach (String s in Directory.GetFiles(Environment.CurrentDirectory, "*.dll"))
        {
            int pos = s.LastIndexOf('\\') + 1;
            Assembly dll = pluginAppDomain.Load(s.Substring(pos, s.Length - pos - 4));
            // Elided... Load types from DLL, etc, etc
        }

Answer 1

Generally you need to unload the AppDomain for the communication.
If you want to prevent the mentioned error you simply can load your dll by using Assembly.Load(Byte[]).

You can also use the Managed Extensibility Framework which will save you a lot of work.

Assembly.Load issue solution

Assembly loaded using Assembly.LoadFrom() on remote machine causes SecurityException