ASP.NET DropDownList 在回发时不保留所选项目
我有一个 ASP DropDownList，它在 Page_Load 事件上填充，在我选择一个项目并单击按钮后，所选项目将被清除，并且 DropDownList 中的第一个项目将被选择。 （只有当页面没有回发时才填充DropDownList）
if (!IsPostBack)
{
List<Country> lCountries = new List<Country>();
List<CompanySchedule> lCompanySchedules = new List<CompanySchedule>();
this.Load_Countries(lCountries);
this.Load_Schedules(lCompanySchedules);
if (personnelRec == null)
{
personnelRec = new Personnel();
}
if (Request.QueryString["UA"] != null && Convert.ToInt32(Request.QueryString["UA"].ToString()) > 0)
{
userAccount.ID = Convert.ToInt32(Request.QueryString["UA"].ToString());
App_Database.Snapshift_Select_Helper.SNAPSHIFT_SELECT_PERSONNEL_APP_ACCOUNT(ref userAccount);
}
this.imgEmployeePicture.ImageUrl = "./images/Employees/nophoto.gif";
if (Request.QueryString["EI"] != null && Convert.ToInt32(Request.QueryString["EI"].ToString()) > 0)
{
this.Load_PersonnelRec(Convert.ToInt32(Request.QueryString["EI"].ToString()));
}
else
{
this.lblChangeDirectionHead.Enabled = false;
this.lblChangeDirections.Enabled = false;
this.lbSchedules.Disabled = true;
}
}
I have an ASP DropDownList that gets populated on the Page_Load event, after i select an item and hit a button the selected item gets cleared and the first item in the DropDownList gets selected. (The DropDownList is only populated when the page is not postback)
if (!IsPostBack)
{
List<Country> lCountries = new List<Country>();
List<CompanySchedule> lCompanySchedules = new List<CompanySchedule>();
this.Load_Countries(lCountries);
this.Load_Schedules(lCompanySchedules);
if (personnelRec == null)
{
personnelRec = new Personnel();
}
if (Request.QueryString["UA"] != null && Convert.ToInt32(Request.QueryString["UA"].ToString()) > 0)
{
userAccount.ID = Convert.ToInt32(Request.QueryString["UA"].ToString());
App_Database.Snapshift_Select_Helper.SNAPSHIFT_SELECT_PERSONNEL_APP_ACCOUNT(ref userAccount);
}
this.imgEmployeePicture.ImageUrl = "./images/Employees/nophoto.gif";
if (Request.QueryString["EI"] != null && Convert.ToInt32(Request.QueryString["EI"].ToString()) > 0)
{
this.Load_PersonnelRec(Convert.ToInt32(Request.QueryString["EI"].ToString()));
}
else
{
this.lblChangeDirectionHead.Enabled = false;
this.lblChangeDirections.Enabled = false;
this.lbSchedules.Disabled = true;
}
}
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页面生命周期执行以下操作（以及与您的问题无关的其他步骤）：
OnInitPage_Load您需要启用 ViewState，以便它可以在“选择”项目之前填充列表。在这种情况下，请确保您不会在 Page_Load 中重新填充并丢失所选值。执行类似
if (!IsPostback) { // Populate }的操作，否则，您必须在每个页面请求的
OnInit事件中手动填充列表。Page_Load在生命周期中为时已晚，因此所选项目会丢失。编辑：
DropDownList还必须设置有效值（与浏览器中显示的文本分开）。这是通过DataValueField属性完成的。每个值必须是唯一的，否则只会选择第一个重复的项目。如果您在浏览器中查看 HTML 源代码，您应该具有：唯一值用于在服务器端选择正确的项目。
The page lifecycle does the following (plus other steps irrelevant to your question):
OnInitPage_LoadYou need to have ViewState enabled so it can populate the list before it "selects" the item. In this case, make sure you don't repopulate in Page_Load and lose the selected value. Do something like
if (!IsPostback) { // Populate }Otherwise, you have to populate the list manually in the
OnInitevent on every page request.Page_Loadis too late in the lifecycle, so the selected item is lost.Edit:
The
DropDownListmust also have valid values set (separate from the text displayed in the browser). This is done through theDataValueFieldproperty. Each value must be unique, otherwise only the first duplicate item will ever be selected. If you look at the HTML source in your browser, you should have:The unique values are used for selecting the right item on the server side.
您使用母版页吗？如果是这样，请记住在母版页中将 EnableViewState 设置为 true。
Are u using a Master Page? If so, remember to put the EnableViewState on true in the master page.