Multithreading concurrency with Interop

[Mark Sisson]

Hey all.
I've got a legacy COM dll that I'd like to use in my multithreaded C#
app. Problem though is when I create several instances of the interop
object and start them on their own threads they always seem to run
synchronously.

I've boiled down the code to a simple "lab" experiment below. In this
console app you'll see that instance one comes back after roughly 4
seconds and then instance two comes back in 8 seconds, twice the time.
This indicates that it is wait on the first thread to finish. Why
won't they run concurrently?

I've tried my best to make sure that the thread types are MTAThread
before calling the COM objects. Also, in VB I made sure the project
threading model was "Apartment Threaded" and the Instancing set to
GlobalUseMulti.

Any ideas???
Thanks

******************* .NET C# CONSOLE TEST APP ********************
using System;
using System.Threading;

namespace ComThreadTest
{
class MyConsoleApp
{
[MTAThread]
static void Main(string[] args)
{
TestClass test1 = new TestClass("1");
TestClass test2 = new TestClass("2");

Thread th = new Thread(new ThreadStart(test1.Go));
th.ApartmentState = ApartmentState.MTA;
th.Start();

th = new Thread(new ThreadStart(test2.Go));
th.ApartmentState = ApartmentState.MTA;
th.Start();
}
}

public class TestClass
{
public DotNetSleeperClass dotnet;
public MyCom.SleeperClass com;
public string mName;

public TestClass(string name)
{
mName = name;
dotnet = new DotNetSleeperClass();
com = new MyCom.SleeperClass();
}

[MTAThread]
public void Go()
{
DateTime start = DateTime.Now;
Console.WriteLine(mName + " OUT at " +
start.ToString("hh:mm:ss:fff"));

//THIS WORK FINE BECAUSE IT'S SIMPLE MANAGED CODE.
//dotnet.Go();

//THIS BLOCKS OTHER THREADS WHILE IT RUNS com.Go();

DateTime finish = DateTime.Now;
TimeSpan ts = finish - start;
Console.WriteLine(mName + " IN at " +
finish.ToString("hh:mm:ss:fff") + " ELAPSED=" +
ts.TotalMilliseconds.ToString() + " milliseconds.");
Console.WriteLine("Press any key to continue...");
Console.Read();
}
}

public class DotNetSleeperClass
{
public DotNetSleeperClass() {}

public void Go()
{
Thread.Sleep(3000);
}
}

}
******************* VB6 ACTIVEX DLL ********************
Option Explicit
Public Sub Go()
'On my box this runs in about 4 seconds to run.
Dim a, b, c
For a = 1 To 1000
For b = 1 To 1000
c = a Mod b
DoEvents
Next b
Next a
End Sub