UDF not used when Excel is programmatically started

I have written a managed Excel UDF. This UDF works well normally, but now I want to start using it in a scenario where no user is involved.

I do something like this:
        private static void OpenExcelFile(string dstFullPathName)
        {
            Excel.Application app = new Microsoft.Office.Interop.Excel.Application();
            try
            {
                object missing = Type.Missing;
                Excel.Workbook wb = app.Workbooks.Open(dstFullPathName, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing);
                wb.Close(false, missing, missing);
            }
            finally
            {
               app.Quit();
            }
        }

There's a script in the workbook that forces recalculation of cells, some of which use my UDF.
However, although the example above seem to work, at least it doesn't crash, it turns out that my UDF is never invoked.

This seems to be because I start Excel programmatically.
If I change
    Excel.Application app = new Microsoft.Office.Interop.Excel.Application();
to
    Excel.Application app = (Microsoft.Office.Interop.Excel.Application) Marshal.GetActiveObject("Excel.Application");
and start Excel manually before I run the code, it works fine. However, this is not a good solution for me, as it requires someone to manually start Excel on the machine after a reboot etc.

Many thanks!
/Fredrik


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