表单提交后，Asp .NET Dropdownlist为空

Question

I have a dropdownlist that is filled dynamically after two textboxes being filled. When I submit the form, the dropdownlist it's empty even after being filled dynamically.

DropDownList

<asp:DropDownList ID="DropDownListReason" runat="server" CssClass="span8">
    <asp:ListItem Text="Select a reason" Value=""></asp:ListItem>
</asp:DropDownList>

Render

 protected override void Render(HtmlTextWriter writer)
 {
     //get the data list.
     foreach (var item in dataList)
     {  
         Page.ClientScript.RegisterForEventValidation(this.DropDownListReason.UniqueID, item.Id.ToString());
     }

     base.Render(writer);
 }

Jquery

//get the data
   $.each(response, function (key, value) {
      if (value.requiresDescription) {
         requiresDescription.push(value.id);
      }

      $("#<%= DropDownListReason.ClientID %>").append($("<option />").val(value.id).text(value.description));
   });

The form submmit action it's a button on click event.

How can I persist the dropdownlist data after the form being submited ?

Answer 1

You need to fill the DropDownList in code behind also. The backed does not know you filled it with javascript, so when you perform a PostBack the page is reloaded and original state is restored, which is no ListItems.

So either add all the options in the DropDownList.

<asp:DropDownList ID="DropDownListReason" runat="server" CssClass="span8">
    <asp:ListItem Text="Select a reason" Value=""></asp:ListItem>
    <asp:ListItem Text="Option 1" Value="1"></asp:ListItem>
    <asp:ListItem Text="Option 2" Value="2"></asp:ListItem>
</asp:DropDownList>

Or add them programatically

<asp:DropDownList ID="DropDownListReason" runat="server" AppendDataBoundItems="true" CssClass="span8">
    <asp:ListItem Text="Select a reason" Value=""></asp:ListItem>
</asp:DropDownList>

DropDownListReason.Items.Insert(DropDownListReason.Items.Count(), new ListItem("Option 1", "1"));
DropDownListReason.Items.Insert(DropDownListReason.Items.Count(), new ListItem("Option 2", "2"));

Now when you submit the page you can read the value of DropDownListReason with DropDownListReason.SelectedValue . This retaining of form values across PostBack is called ViewState . If you're gonna work with webforms you should read up on that.