如何在ASP.NET的Dropdownlist上获取当前项目

Question

My design code :

<asp:DropDownList ID ="DropDownList1" runat="server" EnableViewState="true"  AutoPostBack ="true" CssClass="ddl" DataSourceID="SqlDataSource1" DataTextField="CategoryName" DataValueField="CategoryId" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="SELECT * FROM [Categories]"></asp:SqlDataSource>

C# code:

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Response.Redirect("Products.aspx?catId=" + DropDownList1.SelectedValue);
    }

In database category table:

Book 1
Movie 2
Game 3
Music 4

Book is always current item of drop down list. when i select others, their pages load but dropdownlist's current value is book and i can'response book's page.

Answer 1

I think that the problem is that the list binds with the SqlDataSource on every postback so you are always getting the value of the first item.

To make sure that that's the problem, change the order of the select to DESC and see if you're always getting the last item.

change you're SQL to

SELECT * FROM [Categories] ORDER BY CategoryId DESC

And see if you're always getting 4 .

If that's the problem then you need to bind the list by code always within page load like below:

if(!Page.IsPostback)
{
    // code here
}

Answer 2

You can try two approach Either move the dropdown populating code in the Page_Load and encapsulate the code with

if(!Page.IsPostBack)
{
  //your code
}

or

<%
if(!Page.IsPostBack)
{ %>
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="SELECT * FROM [Categories]"></asp:SqlDataSource>
<%}
%>

Answer 3

Since you are doing a redirect, this means your page is being loaded again with the default binding, making first item selected by default, subscribe to the data bound event and perform selection based on the query string parameter.

ASPX

<asp:DropDownList  OnDataBound="DropDownList1_DataBound" ...

C#

protected void DropDownList1_DataBound(object sender, EventArgs e)
{
   if(!IsPostBack){
     string selectedValue = Request.QueryString["catId"];
     if(!string.IsNullOrEmpty(selectedValue))
       DropDownList1.SelectedValue = selectedValue;
   }
}