// Fix first row
workSheet.Activate();
workSheet.Application.ActiveWindow.SplitRow = 1;
workSheet.Application.ActiveWindow.FreezePanes = true;
// Now apply autofilter
Excel.Range firstRow = (Excel.Range)workSheet.Rows[1];
firstRow.AutoFilter(1, 
                    Type.Missing, 
                    Excel.XlAutoFilterOperator.xlAnd, 
                    Type.Missing, 
                    true);