Weird occasional InvalidCastException in Excel interop

I am developing a fairly comprehensive Excel add-in, applying a lot of ops. Take a workbook, save under new name, apply actions (add names, change formulae, etc).

Generally it works fine, but from time to time .Net throws an InvalidCastException on virtuall开发者_运维问答y any of my CTypes. When I acknowledge the error and run the code again (without quitting Excel) on the exact same Excel workbook, no error occurs. The InvalidCastException occurs maybe in 40% of all tries, otherwise the add-in succeeds.

Recently, I had to convert from C# to VB.Net. I use CType all the time, like Dim interior = CType(cl.Interior, Excel.Interior) (one of the failing statements). The error does not occur in the same place, but all over my program, also when converting ranges, for an For Each r As Excel.Range In largerRange as well as when I use Object in the For Each and CType to Excel.Range. In the latter case, I acquire an Object in the Range but the CType to Excel.Range fails. It there any way to inspect the object at this point?

I really have no clue why the add-in fails one time and the other completes without errors, even without reloading Excel (or any other participating DLL).

Any help would be greatly appreciated!

PS: VS2010, .Net 3.5SP1, Excel 2007, Imports Excel = Microsoft.Office.Interop.Excel

This seems to be a known issue, and as far as I can tell, the official response from Microsoft is "Upgrade to a newer version of .NET".

See this thread on the MSDN forums, which ends with Tom Xu [MSFT] stating:

this issue seems has been fixed in Visual Studio 2010. I think it can be an answer for FerchoArg’s issue.

Martin, I'm no expert, but are you sure you need to use CType so often? For example, here's a simple VSTO addin that manipulates some of the objects you mentioned without casting. It is targeting Excel 2010 and .Net 4.

    Public Class ThisAddIn
        Private Sub ThisAddIn_Startup() Handles Me.Startup
            test()
        End Sub
    End Class

Module Module1
    Sub test()
        Dim appExcel As Excel.Application
        Dim wb As Excel.Workbook
        Dim ws As Excel.Worksheet
        Dim intr As Excel.Interior

        appExcel = Globals.ThisAddIn.Application
        With appExcel
            .Visible = True
            appExcel.ActiveWorkbook.Close(SaveChanges:=False)
            wb = appExcel.Workbooks.Add
            ws = wb.Worksheets(1)
            For Each cell As Excel.Range In ws.Range("A1:B2")
                intr = cell.Interior
                intr.ColorIndex = cell.Row + cell.Column
            Next
        End With
    End Sub
End Module