private void 打开OToolStripButton_Click(object sender, EventArgs e)
        {
            OpenFileDialog openFDlg = new OpenFileDialog();
            openFDlg.InitialDirectory = "C:\\";
            openFDlg.Filter = "文本文件|*.txt|RTF文件|*.RTF|所有文件|*.*";
            openFDlg.FilterIndex = 2;
            openFDlg.RestoreDirectory = true;
            if (openFDlg.ShowDialog()==DialogResult.OK)
            {
                string fName = openFDlg.FileName;
                rtMain.LoadFile(fName);//文件格式无效
                //printDocument
                rtMain.Focus();
                
   
                
            }        }
为什么我打开文件时会出错显示"文件格式无效",应该处么改?注:rtMain是RichTextBox