private void index_Load(object sender, EventArgs e)
        {
            PrintDocument print = new PrintDocument();
            string sDefault = print.PrinterSettings.PrinterName;//默认打印机名
            foreach (string sPrint in PrinterSettings.InstalledPrinters)//获取所有打印机名称
            {
                cbPrinter.Items.Add(sPrint);
                if (sPrint == sDefault)
                    cbPrinter.SelectedIndex = cbPrinter.Items.IndexOf(sPrint);
            }
        }