MarshalByRefObject and Serializable

Recently I ran into a problem with a “serializable” object. After I figured the problem out, I think it deserves to be shared with everyone who is just beginning to learn C# :-P

The case is simple: I have a component which opens a port so that another application can connect to it. I’m developing it, and want to write an integration test in which I start the component in a remote host.

My component is as simple as this:

	AppDomainSetup setup = new AppDomainSetup{ConfigurationFile = "remoteApp.config"};
	var remoteAppDomain = AppDomain.CreateDomain("aremotedomain", null, setup);
	var remoteService = (SimpleComponent)remoteAppDomain.CreateInstanceFromAndUnwrap("SimpleService.dll", "SimpleServiceNameSpace.SimpleComponent");
	Console.Out.WriteLine("Current AppDomain Id: " + AppDomain.CurrentDomain.Id);
	remoteService.Start();

Now I start the component in a remote AppDomain:

	AppDomainSetup setup = new AppDomainSetup{ConfigurationFile = "remoteApp.config"};
	var remoteAppDomain = AppDomain.CreateDomain("aremotedomain", null, setup);
	var remoteService = (SimpleComponent)remoteAppDomain.CreateInstanceFromAndUnwrap("SimpleService.dll", "SimpleServiceNameSpace.SimpleComponent");
	Console.Out.WriteLine("Current AppDomain Id: " + AppDomain.CurrentDomain.Id);
	remoteService.Start();

When running the code, CreateInstanceFromAndUnwrap throws an exception:

	Type 'SimpleServiceNameSpace.SimpleComponent' in assembly 'SimpleService, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.

Uhm, the object is created in the remote AppDomain and is transferred to the CurrentAppDomain, so it makes sense to require the object to be serializable. Make the object serializable? Nothing is as simple as this, is it? I come back to the class and mark it with the “Serializable” attribute.

	[Serializable]
	public class SimpleComponent

Yeah, I get no more error now. I can create an AppDomain and start my component. However, the debug code (Console.Out.WriteLine…) gives me:

Current AppDomain Id: 1

Remote AppDomain Id: 1

Oops, doesn’t it mean that the component is started in the CurrentAppDomain, not in the remote one!?? Well, it’s time to ask my old friend, Google, for help. After searching and looking at sample code which does a similar scenario, I realize that those classes inherit from MarshalByRefObject instead of being marked with the Serializable attribute. Honestly speaking, I read the documentation about MarshalByRefObject many times before but I didn’t get what it was used for :-P In fact, I just understand something when I really need it!!! A quote from MSDN (http://msdn.microsoft.com/en-us/library/system.marshalbyrefobject.aspx):

MarshalByRefObject is the base class for objects that communicate across application domain boundaries by exchanging messages using a proxy. Objects that do not inherit from MarshalByRefObject are implicitly marshal by value. When a remote application references a marshal by value object, a copy of the object is passed across application domain boundaries.

Using MarshalByRefObject, I get:

Current AppDomain Id: 1

Remote AppDomain Id: 2

Blah blah, it works. Briefly speaking: with MarshalByRefObject I get a reference to the remote object via a proxy. When I invoke a method, it is invoked in the remote appdomain via that proxy. On the contrary, with the Serializable attribute, a copy of the remote object is made in the current app domain. Thus, the method is invoked in the current app domain as well.