SaveFileDialog dlg = new SaveFileDialog();
            if (dlg.ShowDialog() == DialogResult.OK)
            {
                MessageBox.Show(dlg.FileName);
            }

------解决方案--------------------
            SaveFileDialog sfd = new SaveFileDialog();
           DialogResult dr = new DialogResult();
           dr=sfd.ShowDialog();
           if (dr == DialogResult.OK)
           {
               文件名=sfd.FileName;  
               下面是处理文件
           }



------解决方案--------------------
 SaveFileDialog dlg = new SaveFileDialog();
           if (dlg.ShowDialog() == DialogResult.OK)
           {
              Streamwrite sw =  File.CreateText(dlg.fullname);
               然后把文件内容写入流，最后关闭sw.close();
           }