C#-DropDownList导致错误：对象引用未设置为对象的实例

我不断收到此错误,不知道为什么.我搜索并扫描了asp.net网站两天了,因此我需要一些帮助.错误是：

Object reference not set to an instance of an object

这是我的代码：

DropDownList DropDownList1 =(DropDownList)ListView1.InsertItem.FindControl("DropDownList1");

string highToLow = DropDownList1.SelectedValue;
string lowToHigh = DropDownList1.SelectedValue;

if (highToLow == "1")
{
    var exmapleFilter = from users in testEntities.users 
                        orderby users.id descending
                        select users;

    ListView1.DataBind();
}

我将下拉列表的值设置为1(高)和2(低),更改了选定的索引后,我想运行ADO.net实体框架代码以返回排序的数据列表.

我目前正在使用linq数据源和列表视图来显示数据库中的内容.

谢谢.

编辑：

这是堆栈跟踪

   System.NullReferenceException was unhandled by user code
   Message=Object reference not set to an instance of an object.
   Source=App_Web_s0ked5y3

   StackTrace:
   at Default.DropDownList1_SelectedIndexChanged(Object sender, EventArgs e)
   in Default:line 120

   at System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e)

   at System.Web.UI.WebControls.DropDownList.RaisePostDataChangedEvent()

   at System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.
   RaisePostDataChangedEvent()

   at System.Web.UI.Page.RaiseChangedEvents()

   at System.Web.UI.Page.ProcessRequestMain(Boolean 
   includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   InnerException: 

解决方法:

检查您是否在列表视图中

<InsertItemTemplate>
   ....          
   <asp:DropDownList ID="DropDownList1" runat="server" />
   .... 

与…关联

ListView1.InsertItem.FindControl("DropDownList1");

标签：visual-studio-2010,drop-down-menu,asp-net,c,entity-framework
来源： https://codeday.me/bug/20191201/2078291.html