未将对象引用设置到对象的实例 怎么解决
做了一个 修改已经上传音乐的页面。在写绑定信息是 string _id = Request.QueryString["id"].ToString().Trim();这句总是提示 未将对象引用设置到对象的实例。请问怎么解决。
代码如下：
protected void PageInit()
  {
  string _id = Request.QueryString["id"].ToString().Trim(); _id = BasePage.CheckStr(_id);
  tbMusicDal tbMusicDal1 = new tbMusicDal();
  DataSet ds1 = tbMusicDal1.GetList(string.Format("id={0}", _id));
  if(ds1.Tables[0].Rows.Count > 0)
  {
  DataRow dr = ds1.Tables[0].Rows[0];
  tbName.Text = dr["Name"].ToString().Trim();
  tbGroupName.Text = dr["Name"].ToString().Trim();
  tbAuthors.Text = dr["Authors"].ToString().Trim();
  tbLyrics.Text = dr["Contents"].ToString().Trim();
  lb_path.Text = dr["MusicUrl"].ToString().Trim();
  //绑定下拉列表
  string strSql = "select * from tbMusicClass";
  SqlDataAdapter sda = new SqlDataAdapter(strSql, BasePage.GetConn());
  DataSet ds = new DataSet();
  sda.Fill(ds, "tbMusicClass");
  string _html = "<select id=\"selectClass\" name=\"selectClass\">";
  foreach (DataRow draaa in ds.Tables[0].Rows)
  {
  if (dr["ParentId"].ToString().Trim() == draaa["id"].ToString().Trim())
  {
  _html += string.Format("<option value=\"{0}\" selected=\"selected\">{1}</option>", draaa["id"].ToString(), draaa["ClassName"].ToString());
  }
  else
  {
  _html += string.Format("<option value=\"{0}\">{1}</option>", draaa["id"].ToString(), draaa["ClassName"].ToString());
  }
  }
  _html += "</select>";
  ltr_class.Text = _html;
  }
  }

------解决方案--------------------
string _id =string.Empty;

if (!string.IsNullOrEmpty(Request.QueryString["id"]))
_id = Request.QueryString["id"].ToString().Trim();
------解决方案--------------------
string _id=string.Empty;
if (null != Request.QueryString["id"])
{
_id = Request.QueryString["id"].ToString().Trim();
}