Excel file operations using interop in multithreaded C# application fails

I've an application that automates some file related jobs. Every job is executed inside separate threads. One kind of job is exporting an Excel file to HTML format. I use Microsoft.Office.Interop.Excel namespace for this purpose. My application was working fine under Windows Server 2008 environment but we upgraded our server to Windows Server 2012 and I started to get the following error :

The message filter indicated that the application is busy. (Exception from HRESULT: 0x8001010A (RPC_E_SERVERCALL_RETRYLATER))

The thing is first call to export function successfully exports Excel file to HTML but successive calls fails with the above error. I make sure to close and finalize all my Excel related objects and check from task manager that excel.exe is not working but with no luck.

I use the following code to retry if this error occurs but it keeps getting the exception and fails after 5 retries

while (!success)
            {
try
                {
                    ExportExcel();
                    success = true;
                    System.Threading.Thread.Sleep(2000);
                }
                catch (System.Runtime.InteropServices.COMException loE)
                {
                    tryCount++;
                    if (loE.HResult.ToString("X") == "80010001" || loE.HResult.ToString("X") == "8001010A" && tryCount<5)
                    {                                                                     
                      System.Threading.Thread.Sleep(2000);
                    }
                    else
                    {
                        throw;
                    }
                }
             }

I suspect this might be something related some threading error but I can't come up with an answer. Any insight would be helpful.

Thank you Joe for pointing out the right way:

I ended up using a solution with a mixture of the following links: http://blogs.artinsoft.net/Mrojas/archive/2012/09/28/Office-Interop-and-Call-was-rejected-by-callee.aspx

http://blogs.msdn.com/b/pfxteam/archive/2010/04/07/9990421.aspx

So I used something like the following:

StaTaskScheduler cts=new StaTaskScheduler(1);
TaskFactory factory;          
factory = new TaskFactory(cts);
Task jobRunTask = factory.StartNew(() =>
{
   MessageFilter.Register();
   ExcelInteropFunction();
   MessageFilter.Revove();
 });

I believe the Excel object model is apartment threaded, so calls from your multiple threads will be marshaled to the same thread in the Excel process - which may be busy especially if there are several client threads.

You can implement IMessageFilter (OLE message filter, not to be confused with System.Windows.Forms.IMessageFilter) to provide custom retry logic.

Your server upgrade might have changed the timing characteristics so that the problem occurs more frequently.

UPDATE

Here is a sample basic implementation of an OLE message filter:

    // Definition of the IMessageFilter interface which we need to implement and 
    // register with the CoRegisterMessageFilter API.
    [ComImport(), Guid("00000016-0000-0000-C000-000000000046"), InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]
    interface IOleMessageFilter // Renamed to avoid confusion w/ System.Windows.Forms.IMessageFilter
    {
        [PreserveSig]
        int HandleInComingCall(int dwCallType, IntPtr hTaskCaller, int dwTickCount, IntPtr lpInterfaceInfo);
        [PreserveSig]
        int RetryRejectedCall(IntPtr hTaskCallee, int dwTickCount, int dwRejectType);
        [PreserveSig]
        int MessagePending(IntPtr hTaskCallee, int dwTickCount, int dwPendingType);
    }

    internal sealed class OleMessageFilter : IOleMessageFilter, IDisposable
    {
        [DllImport("ole32.dll")]
        private static extern int CoRegisterMessageFilter(IOleMessageFilter newFilter, out IOleMessageFilter oldFilter);

        private bool _isRegistered;
        private IOleMessageFilter _oldFilter;

        public OleMessageFilter()
        {
            Register();
        }

        private void Register()
        {
            // CoRegisterMessageFilter is only supported on an STA thread.  This will throw an exception
            // if we can't switch to STA
            Thread.CurrentThread.SetApartmentState(ApartmentState.STA);

            int result = CoRegisterMessageFilter(this, out _oldFilter);
            if (result != 0)
            {
                throw new COMException("CoRegisterMessageFilter failed", result);
            }
            _isRegistered = true;
        }

        private void Revoke()
        {
            if (_isRegistered)
            {
                IOleMessageFilter revokedFilter;
                CoRegisterMessageFilter(_oldFilter, out revokedFilter);
                _oldFilter = null;
                _isRegistered = false;
            }
        }

        #region IDisposable Members

        private void Dispose(bool disposing)
        {
            if (disposing)
            {
                // Dispose managed resources
            }
            // Dispose unmanaged resources
            Revoke();
        }

        void IDisposable.Dispose()
        {
            GC.SuppressFinalize(this);
            Dispose(true);
        }

        ~OleMessageFilter()
        {
            Dispose(false);
        }

        #endregion

        #region IOleMessageFilter Members

        int IOleMessageFilter.HandleInComingCall(int dwCallType, IntPtr hTaskCaller, int dwTickCount, IntPtr lpInterfaceInfo)
        {
            return 0; //SERVERCALL_ISHANDLED
        }

        int IOleMessageFilter.RetryRejectedCall(IntPtr hTaskCallee, int dwTickCount, int dwRejectType)
        {
            if (dwRejectType == 2) // SERVERCALL_RETRYLATER
            {
                return 200; // wait 200ms and try again
            }

            return -1; // cancel call
        }

        int IOleMessageFilter.MessagePending(IntPtr hTaskCallee, int dwTickCount, int dwPendingType)
        {
            return 2; //PENDINGMSG_WAITDEFPROCESS
        }
        #endregion
    }

You can also look at this sample, though it displays a prompt asking the user if they want to retry, which is probably not appropriate if your client is multithreaded and server-based.