Auto-populated Listbox based on selection from other Listbox

I'm trying to auto-populate a listbox of zip codes from a database table state_zipcode_plans based on the state a user selects from the previous listbox. I'm getting the error: Must declare the scalar variable @state_code.
Any suggestions on fixing this?
Can it be fixed by adding in Request.Form("state_code"

) in somewhere?

Here's my code:

<asp:ListBox ID="ST_Select_ListBox" runat="server" DataSourceID="SqlDataSource1"

DataTextField="state_code" DataValueField="state_code" CssClass="style7"></asp:ListBox

<asp:SqlDataSource ID="SqlDataSource1" runat="server"

ConnectionString="<%$ ConnectionStrings:PriceFinderConnectionString %>"

SelectCommand

="SELECT [state_code] FROM [state_code] ORDER BY [state_code]">

<SelectParameters

<asp:FormParameter DefaultValue="NULL" FormField="state_code" Name="state_code" Type="String"

</SelectParameters

asp:SqlDataSource

<br class="style7"

</td

Select Zip (auto-populated based on selected State)

</span

<br class="style7"

<asp:ListBox ID="Zip_Select_ListBox" runat="server" DataSourceID="SqlDataSource2" CssClass

="style7">

</asp:ListBox

<asp:SqlDataSource ID="SqlDataSource2" runat="server"

ConnectionString="<%$ ConnectionStrings:PriceFinderConnectionString %>"

SelectCommand="SELECT DISTINCT [zip_code] FROM [state_zipcode_plans] WHERE ([state_code] = @state_code)" Request.Form("state_code")

<SelectParameters

<asp:FormParameter DefaultValue="NULL" FormField="zip_code" Name="zip_code" Type="String"

</SelectParameters

</asp:SqlDataSource

Re: Auto-populated Listbox based on selection from other Listbox

Whenever you define parameters in SelectCommand, parameters should be defined in the SelectParameters as well..

<asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
    SelectCommand="SELECT DISTINCT [zip_code] FROM [state_zipcode_plans] WHERE ([state_code] = @state_code)">
    <SelectParameters>
        <asp:ControlParameter ControlID="ST_Select_ListBox" Name="state_code" PropertyName="SelectedValue" />
    </SelectParameters>
</asp:SqlDataSource>

Re: Auto-populated Listbox based on selection from other Listbox

how is that different from what i had before?

Re: Auto-populated Listbox based on selection from other Listbox

Only difference is adding the select parameter for the SqlDataSource to pass the State ID from the DropDownlist selection to filter Districts.