Question

I have an excel addin class that has this function:

public void testRibbon()
{
     Excel.Workbook workbook = this.Application.ActiveWorkbook;      
     Excel.Worksheet activeSheet = workbook.Sheets[1];
     Excel.Range range = activeSheet.get_Range(JOB_ID_FIELD + HEADER_ROW, TOTAL_STATUS_PERCENTAGE_KEY_FIELD + (10 + 1).ToString());
     range.get_Range("C4").Value = "test Ribbon complete";
}

In the ribbon I added a button that when pressed will call testRibbon in a thread:

private void process_Click(object sender, RibbonControlEventArgs e)
{
    Thread processThread = new Thread(delegate(){
        Globals.ExcelAddin.testRibbon();
    });
    processThread.Start();
} 

This causes a cast error:

Unable to cast COM object of type ‘System.__ComObject’ to interface
type ‘Microsoft.Office.Interop.Excel._Workbook’. This operation failed
because the QueryInterface call on the COM component for the interface
with IID ‘{000208DA-0000-0000-C000-000000000046}’ failed due to the
following error: Error loading type library/DLL. (Exception from
HRESULT: 0x80029C4A (TYPE_E_CANTLOADLIBRARY)).

The cast error doesn’t happen if I don’t use a new thread.

Edit:Tried using task factory, same error:

var task2 = Task.Factory.StartNew(
() =>
{
     Globals.BobStats.testRibbon();
});

Answer 1

Using multiple threads in Excel makes no sense because Excel’s COM interface is basically single threaded (see this article). There might be further restrictions from within an add-in.

Yet I have to say I don’t really understand what the error message is trying to say.