xml文件的有关问题

求助xml文件的问题
string file = HttpContext.Current.Server.MapPath("~/file/count.xml");
  public int add(string sname,string spass,string dt)
  {
  HyperLink1.NavigateUrl = file;
   
  int cid=0;
  try{
  XmlDocument xdl=new XmlDocument();
   
  xdl.Load(file);
  XmlNode node=xdl.SelectSingleNode("/counts");
  if(node!=null)
  {
  XmlNode last=node.LastChild;
  cid=Int32.Parse(last.Attributes["id"].Value)+1;
  }
  else
  {
  cid=1;
  }
  XmlNode child=xdl.CreateNode(XmlNodeType.Element,"count",null);
  addattribute(xdl,child,"id",cid.ToString());
  addattribute(xdl, child, "name", sname);
  addattribute(xdl,child,"passWord",spass);
  addattribute(xdl, child, "dateTime", dt);
  node.AppendChild(child);
  xdl.Save(file);
  }
  catch (Exception ex)
  {
  throw new Exception(ex.Message);
  }
  return cid;
   
  }
  private void addattribute(XmlDocument xmldoc, XmlNode parent, string sname, string value)
  {
  XmlAttribute attribute = xmldoc.CreateAttribute(sname, null);
  attribute.Value = value;
  parent.Attributes.Append(attribute);
  }





  protected void Button1_Click(object sender, EventArgs e)
  {
  try
  {
  add(TextBox1.Text, TextBox2.Text, System.DateTime.Now.ToString());
  }
  catch (Exception ex)
  {
  throw new Exception(ex.Message);
  }
  }
可是 提交总是说无法将对象引入到所制订的实例？

[解决办法]
Object reference not set to an instance of an object.
是这个错误吗?
代码: 

C# code

cid=Int32.Parse(last.Attributes["id"].Value)+1;
[解决办法]
把具体的错误是哪一条语句产生的，描述一下