Hi taj,

QueryInterface gets automatically called when you cast
to one of the interfaces.

Is shellFolder shared between the threads? I mean: do you call
SHGetDesktopFolder(out shellFolder) in your main thread
and shellIcon = shellFolder as IShellIcon in the secondary
thread?

If the COM object behind the IShellIcon interface is not
free-threaded, then QueryInterface will fail.

Rob

Hi Robert, hi all,

i spent more investigation into this... i found out, that when releasing
the IShellFolder interface in the main thread before it is used in the
second one, the second one is able to use it...
if you uncomment out the line "Marshal.ReleaseComObject", the thread
(which starts after the main thread finished) is able to do the
transformation into IShellIcon; if not, it fails.

it's still magic that the original SHGetDesktopFolder seems to work,
just the casting into IShellIcon fails; i don't think this is interface
specific behaviour, cause i already saw and used SHGetDesktopFolder in
multi-threaded environments very well.

i post the (really reduced) code which may be used to reproduce the
problem, as mentioned uncomment the Marshalling-code to see it is very
well able to work somehow...

Robert, i will be gone for business travel till friday, i would be glad
if you find the time to give it one more minute; perhaps you find the
time and we will be able to continue the discussion of this topic then.

Regards & thousands thanks,
taj

// Console application that reproduces the topic
using System;
using System.Threading;
using System.Runtime.InteropServices;

namespace ConsoleApplication1 {
 class Class1 {
  [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
   Guid("000214E5-0000-0000-C000-000000000046")]
   public interface IShellIcon {
  }

  [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
   Guid("000214E6-0000-0000-C000-000000000046")]
   public interface IShellFolder {
  }

  [STAThread]
  static void Main(string[] args) {
   IShellFolder shellFolder = null;
   SHGetDesktopFolder(out shellFolder);

   Thread mythread = new Thread(new ThreadStart(MyThreadWorker));
   mythread.ApartmentState = ApartmentState.STA;
   ApartmentState aps = mythread.ApartmentState;
   mythread.Start();

   //Marshal.ReleaseComObject(shellFolder);
   mythread.Join();
  }

  [DllImport("shell32.dll")]
  public static extern Int32 SHGetDesktopFolder(out IShellFolder ppshf);

  public static void MyThreadWorker() {
   try {
    IShellIcon shellIcon = null;
    IShellFolder shellFolder = null;

    SHGetDesktopFolder(out shellFolder);
    shellIcon = shellFolder as IShellIcon;

    if(shellIcon != null)
     System.Windows.Forms.MessageBox.Show("Got it!");
   } catch(Exception ex) {
    string test = ex.Message;
   }
  }
 }
}