I've ran into a problem where the sheet.Copy() method crashes saying "Interface not registered" I am trying to copy a worksheet from one workbook to another opened up one. This is the code (it is inside a For loop):
xlWorkbook3 = xlApp3.Workbooks.Open(finalDir);
Excel.Worksheet xl = xlWorkbook3.Sheets[1];
xl.Copy(Before: xlWorksheet2);
Marshal.ReleaseComObject(xl);