Ref argument through a RealProxy

Question

I need to call a method with ref-arguments through a RealProxy. I have isolated the problem down to the following code:

using System;
using System.Reflection;
using System.Runtime.Remoting.Messaging;
using System.Runtime.Remoting.Proxies;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
             HelloClass hello=new HelloClass();
             TestProxy proxy = new TestProxy(hello);
             HelloClass hello2 = proxy.GetTransparentProxy() as HelloClass;

             string t = "";
             hello2.SayHello(ref t);
             Console.Out.WriteLine(t);
        }
    }

    public class TestProxy : RealProxy
    {
         HelloClass _hello;

         public TestProxy(HelloClass hello)
             : base(typeof(HelloClass))
         {
             this._hello = hello;
         }

         public override System.Runtime.Remoting.Messaging.IMessage Invoke(System.Runtime.Remoting.Messaging.IMessage msg)
        {
            IMethodCallMessage call = msg as IMethodCallMessage;
            object returnValue = typeof(HelloClass).InvokeMember(call.MethodName, BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Instance, null, _hello, call.Args);
            return new ReturnMessage(returnValue, null, 0, call.LogicalCallContext, call);
        }
    }

    public class HelloClass : MarshalByRefObject
    {
        public void SayHello(ref string s)
        {
            s = "Hello World!";
        }
    }
}

The program should produce the "Hello World!" output, but somehow the modification of the ref arguments gets lost in the proxy. What do I have to do to get this to work?

Answer 1

The second parameter of ReturnMessage needs to contain the values of the ref and out parameters to pass back. You can get them by saving a reference to the array you pass in:

public override System.Runtime.Remoting.Messaging.IMessage Invoke(System.Runtime.Remoting.Messaging.IMessage msg)
{
    IMethodCallMessage call = msg as IMethodCallMessage;
    var args = call.Args;
    object returnValue = typeof(HelloClass).InvokeMember(call.MethodName, BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Instance, null, _hello, args);
    return new ReturnMessage(returnValue, args, args.Length, call.LogicalCallContext, call);
}