setting pasword on a excel file

Try this

public void SaveFile()

        {

            this.excelWorkbook.SaveAs(

                this.excelWorkbook.FullName,

                vk_format,

                "",

                vk_write_res_password,

                vk_read_only,

                null,

                 Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange,

                 null,

                 vk_add_to_mru,

                 null,null,vk_local);

        }