c#编程，怎么对文本文档中字符进行加密，方法是用ASCLL值减10

string fileName = AppDomain.CurrentDomain.BaseDirectory + "Exam1.txt";
            FileStream fs = new FileStream(fileName, FileMode.Create, FileAccess.ReadWrite);
            StreamWriter sw = new StreamWriter(fs);
            string Str = "";
            foreach (char c in fileName.ToCharArray())
            {
                Str += Encoding.ASCII.GetString(new byte[] { (byte)(c-10) });
            }
            sw.Write(Str);
            MessageBox.Show("加密成功！");
            sw.Close();
            fs.Close();

你这个只是加密了文件名，得考虑文件内容的长度，加密，然后写入！