问一下关于邮件客户端的问题

  private string QPUnEncryCode(string source)
        {
            source=source.Replace ("=\r\n","");
             int len=source.Length ;
             string dest=string.Empty ;
            int i=0;
            while(i<len)
            {
                string temp=source.Substring (i,1);
               if(temp=="=")
                {
                    int code=Convert.ToInt32 (source.Substring (i+1,2),16);
                    if(Convert.ToInt32 (code.ToString (),10)<127) 
                   {
                        dest+=((char)code).ToString ();
                        i=i+3;
                    }
                    else  
                    {
                        dest+=System.Text.Encoding.Default.GetString(new byte []{Convert.ToByte  (source.Substring (i+1,2),16),Convert.ToByte  (source.Substring (i+4,2),16)}) ;
                        i=i+6;
                    }
                }
                else
                {
                    dest+=temp;
                    i++;
                }
            }
            return dest;
        }