大家帮看下打印功能实现错误

Question

       private void 打印ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            //管理打印预处理
            printDocument.BeginPrint += new PrintEventHandler(printDocument_BeginPrint);
             //关联打印代码
            printDocument.PrintPage += new PrintPageEventHandler(printDocument_PrintPage);
             //定义打印对话框
            PrintDialog printDialog = new PrintDialog();
             //获得用户输入
            DialogResult dr = PrintDialog.ShowDialog();
             //如果确认则打印
            if (dr==DialogResult.OK)/*Error An object reference is required for the non-static field, method, or property 'System.Windows.Forms.CommonDialog.ShowDialog()'*/            {
                printDocument.Print();
            }
            else
            {
                return;
            }
        }
        /// <summary>
        /// 打印前预处理代码
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void printDocument_BeginPrint(object sender, PrintEventArgs e)
        {
            //设置打印文本
            text = richTextBox1.Text;
        }
        /// <summary>
        /// 打印文档代码
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void printDocument_PrintPage(object sender, PrintEventArgs e)
        {
            //用DrawString方法输出字符串
            e.Graphics.DrawString(text,new Font("Arial",10),Brushes.Black,20,20);/*'System.Drawing.Printing.PrintEventArgs' does not contain a definition for 'Graphics' and no extension method 'Graphics' accepting a first argument of type 'System.Drawing.Printing.PrintEventArgs' could be found (are you missing a using directive or an assembly reference?)*/
        }