Fast Marshaling for Cross-AppDomain Proxies

Introduction 

This article briefly describes how to increase the speed while marshaling across application domains. The intention of this how-to is to provide some tips and some clues about this issue.

Background 

Fast cross-AppDomain calls were a need for me, while trying to implement a plug-in architecture for my application. The one problem of performance I've faced was that cross application domain method calls were 1000x slower than standard method calls. 

Using the Code

The solution has three projects: one for the application executable, another one for the "PluginInterface", and another for the plugin itself. Both projects, the plugin and the executable reference the PluginInterface. That's because when executing your program, referenced libraries get loaded automatically in the default AppDomain, and then, it's impossible to unload them. By creating an intermediate DLL (the PluginProxy) we can create an AppDomain in which types loaded by this DLL remain isolated from the default AppDomain, giving us the possibility to unload previously loaded plugins, using AppDomain.Unload(...).

To describe how to improve this situation, let's define some classes to use as an example.

The plugin class. This class should be in an independent assembly, like "Plugin.dll". The catch: it is ~1000x slower to invoke a method for an object instance residing in another application domain.

public class Plugin : MarshalByRefObject, PluginInterface.IPlugin
{
    public string SendMessage(string message)
    {
        return "Received: " + message;
    }
}

Then the IPluginProxy and the IPlugin interfaces should also be in another independent assembly, like "PluginInterface.dll". For the sake of simplicity of this article, IPluginProxy's implementation is left as an exercise to the reader.

public interface IPlugin
{
    string SendMessage(string message);
} 

public interface IPluginProxy
{
    IPlugin Create(AssemblyName assemblyName);
}	 

Last but not least, the plug-in consumer, your executable file. The first tip for speeding up marshaling is right here, in your application's entry point.

[STAThread, LoaderOptimization(LoaderOptimization.MultiDomainHost)]
static void Main()
{
    AppDomain pluginDomain = AppDomain.CreateDomain("PluginDomain");

    [...]  // you should create an instance of the proxy in the pluginDomain AppDomain.
    [...]  // Then, by calling proxy.Create() the proxy should create an instance
    [...]  // of the plug-in in that AppDomain, so it can be unloaded later if necessary.

    PluginInterface.IPlugin plugin = proxy.Create(pluginAssembly);

    plugin.SendMessage("hello from another AppDomain");  // if all goes well, 
					// this call should be 10x faster
}

Now, for this to work faster, first it's necessary to:  

1. Apply a strong name to PluginInterface.dll and to the plugin(s), in this example, "Plugin.dll".
2. Install PluginInterface.dll and Plugin.dll into the GAC.
3. The entry point of your application should have the following attribute applied:
   C#

   LoaderOptimization(LoaderOptimization.MultiDomainHost)  

And... that's it. If you followed those steps, and your plugin is getting loaded in another domain, other than the default domain, then, you should see some improvements, like this:  

History

• 24^th April, 2009: Initial post