foreach(GridViewItem gvi in this.gv_IsPopular.Items)
{
LinkButton up = (LinkButton)gvi.FindControl("linkbtn_Up"); 
LinkButton down = (LinkButton)gvi.FindControl("linkbtn_down"); 
if (whetherLine == 1) 
{ 
      up.Enabled = false; 
 } 
 whetherLine++; 
 if (whetherLine == gvi.Count) 
 { 
      down.Enabled = false; 
 } 
}

------解决方案--------------------
linkbtn_Up是在ItemTemplate还是EditItemTemplate中的，如果是在EditItemTemplate中，楼主这样得不到

------解决方案--------------------
把代码帖完整
linkbtn_Up是在ItemTemplate还是EditItemTemplate中

------解决方案--------------------
LinkButton up =e.Row.FindControl("linkbtn_Up") as LinkButton;
up肯定没有找到，最好在使用之前判断一下是否null然后再处理
否则就会"未将对象引用设置到对象的实例 "


------解决方案--------------------
把ASPX 页代码贴一下

顺便把代码也改一下
C# code

            LinkButton up =e.Row.FindControl("linkbtn_Up") as LinkButton; 
            LinkButton down = e.Row.FindControl("linkbtn_down") as LinkButton; 
            if (whetherLine == 1) 
            { 
                if(up!=null)
                  up.Enabled = false; 
            } 
            whetherLine++; 
            if (whetherLine == a.Rows.Count) 
            { 
                if(down!=null)
                    down.Enabled = false; 
            }