datagrid 排序:给标题加图标

private void DataGrid1_SortCommand(object source, DataGridSortCommandEventArgs e)
  {
   #region 设置排序列的图标及排序状态(通用的代码)
   DataGrid dg = (DataGrid)source;

   string strImg="<img src="+Request.ApplicationPath+"/icons/sort_down.gif>";
   string strImgUp="<img src="+Request.ApplicationPath+"/icons/sort_up.gif>";

   string sortkey=e.SortExpression;

   int colindex = -1;//获取所点击列的column index
   for(int i=0;i<dg.Columns.Count;i++)
   {
    if(dg.Columns[i].SortExpression == e.SortExpression)
    {
     colindex = i;
     break;
    }
   }   
   if(ViewState["sorttype"]!=null && (String)ViewState["sorttype"]!=String.Empty)//记录排序状态
   {
    ViewState["sorttype"]="";
   }
   else
   {
    ViewState["sorttype"]=" desc";
   }
   sortkey+=ViewState["sorttype"];
   ViewState["SortKey"]=sortkey;

   //清除所有列的排序图际
   for(int i=0;i<dg.Columns.Count;i++)
   {
    dg.Columns[i].HeaderText=(dg.Columns[i].HeaderText).ToString().Replace(strImg,"");
    dg.Columns[i].HeaderText=(dg.Columns[i].HeaderText).ToString().Replace(strImgUp,"");
   }

   if(ViewState["sorttype"].ToString().Trim()=="")
   {
    strImg=strImgUp;
   }
   dg.Columns[colindex].HeaderText=dg.Columns[colindex].HeaderText + strImg;
   #endregion 设置排序列的图标及排序状态
   getdata(ViewState["SortKey"].ToString());//获取数据
  }