Hi again,

I'm unable to create such short example, but I've tried to copy/paste the
parts of where the threading problem occurs. There are only three classes
involved, I've added comments to describe what I've tried to do.

////////////////////////////////////////////////////////////////////////
// Code in the main thread
// This code start the entire processing of the reports. This is a Form
class instance.
// The exception message should end up here, so it can be shown.
////////////////////////////////////////////////////////////////////////

private void btnPrint_Click(object sender, EventArgs e)
{
   ReportsGateway.CreateReport(userLaen, arnIDs, ardIDs, true, true, this);
}

// I've tried having this callback method in the child thread, but did not
work.
// I would actually rather have it in the child thread, because others might
start
// the report processing in the future.
public void ShowThreadError(string text)
{
   throw new BofException("Error generating reports." + Environment.NewLine
+ Environment.NewLine + text);
}

////////////////////////////////////////////////////////////////////////
// Code in the report helper (gateway), an in-between layer.
// This is where the child thread is created. This is a static class,
// inheriting from Object
////////////////////////////////////////////////////////////////////////

public static void CreateReport(int userLaen, int[] arnIDs, int[] ardIDs,
   bool onlyPreview, bool printCase, MyBaseForm callingForm)
{
   CreateMultiReport(userLaen, arnIDs, ardIDs, callingForm);
}

private static void CreateMultiReport(int userLaen, int[] arnIDs, int[]
ardIDs, MyBaseForm callingForm)
{
   object[] agOb = new object[4];

   agOb[0] = userLaen;
   agOb[1] = arnIDs;
   agOb[2] = ardIDs;
   agOb[3] = callingForm;

   Thread t = new Thread(ReportGenerationForm.StartProcessingReports);
   t.SetApartmentState(ApartmentState.STA);
   t.IsBackground = true;

   ReportGenerationForm.paramObjects = agOb;

   t.Start();
}

////////////////////////////////////////////////////////////////////////
// Code in the child thread
// An exception might occur here, that should be pushed up to the main
// UI thread, for nomal display. This is a Form class instance.
////////////////////////////////////////////////////////////////////////

// I've tried to declare this in the main form gui, but no difference.
public delegate void ShowErrorMessage(string text);

public static void StartProcessingReports()
{
   ReportGenerationForm frm = new
ReportGenerationForm((int)paramObjects[0],
       paramObjects[1] as int[], paramObjects[2] as int[], paramObjects[3]
as MyBaseForm, true, true, true);
   IntPtr handle = frm.Handle;
   frm.ShowPrint();
}

private void ShowPrint()
{
   try
   {
       ShowDialog();
       TopMost = true;
       BringToFront();
   }
   catch (Exception ex)
   {
     // I've tried lots of different things here, for example calling
Invoke on
     // the delegate, calling invoke on the calling form etc etc, but no
use.
       ShowErrorMessage d = new ShowErrorMessage((_callingForm as
XXX.YYY.ReportMainthreadForm).ShowThreadError);
       IntPtr handle = this.Handle;
       this.Invoke(d, new string[] { ex.Message });
   }
}

regards

Carl