Question

I’ve written a very basic web server in C# that loads custom modules that handle requests to a specific domain name, as specified in a config file. The custom modules are loaded into a new AppDomain because I need the ability to unload them dynamically (good for security, too). Because modules are loaded into a new AppDomain all parameters and return types are MarshalByRefObject. This works fine and I pass a HttpRequest object that inherits from MarshalByRefObject and return a LinkedList that is sent back to the client by the web server.

All of this works well, but a lot of the data is passed as byte[] and I believe the proxy for MarshalByRefObject will copy all of the bytes from the new AppDomain to the main AppDomain instead of accessing them directly. So, if I’m right about this, if one of the modules would send a 5MB file as a response then 5MB would be loaded/generated in the module, then copied from the modules AppDomain to the main AppDomain and finally sent through the socket back to the client.

So, my question is: can I get around this somehow so it doesn’t copy so much data between AppDomain’s? Or is there a better way to do this that doesn’t use MarshalByRefObject?

Answer 1

I ended up passing the SocketInformation to the new AppDomain and creating a new socket object there (in the new AppDomain).

In my “Sandbox wrapper object” I have this function:

internal class Sandbox
{
    private AppDomain _AppDomain;
    private WebApplicationProxy _Proxy;

    public Sandbox(string assemblyFile)
    {
        _AppDomain = AppDomain.CreateDomain(Guid.NewGuid().ToString());
        _Proxy = (WebApplicationProxy)_AppDomain.CreateInstanceAndUnwrap(
                                                    Assembly.GetExecutingAssembly().FullName,
                                                    "WebServer.WebApplication.Proxy");
        _Proxy.Initialize(assemblyFile);
    }
    public void SendResponse(Socket client, HttpRequest request)
    {
        SocketInformation clientInfo = client.DuplicateAndClose(Process.GetCurrentProcess().Id);
        _Proxy.GetResponse(clientInfo, request);
    }
}

And in the new AppDomain “resumes” the socket, I’m not sure yet what actually happens behind the scenes… here’s the code:

internal class Proxy : MarshalByRefObject
{
    private AppController _AppController;

    public void Initialize(string assemblyFile)
    {
        Assembly asm = Assembly.LoadFile(assemblyFile);
        var q = from t in asm.GetTypes()
                where t.GetInterfaces().Contains(typeof(AppController))
                      && !t.IsAbstract && t.IsClass
                select t;
        foreach (Type t in q)
        {
            _AppController = (AppController)Activator.CreateInstance(t);
        }
    }

    public void SendResponse(SocketInformation clientInfo, HttpRequest req)
    {
        Socket client = new Socket(clientInfo);

        LinkedList<byte[]> toSend = _AppController.GetResponse(client, req);

        foreach (byte[] bytes in toSend)
            client.Send(bytes);

        client.Close();
    }
}

I’m not sure about the details of what happens when DuplicateAndClose is invoked, or creating the new Socket object in the new domain, but it is working well so far.