Question

I have 2 dropdownlists, one is State and the second is City. I am trying to create it so, when a user clicks the State, the second dropdownlist is populated with City names from a datatable.

Here is the code

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<%--<cc2:CascadingDropDown ID="CascadingDropDown1" runat="server"> </cc2:CascadingDropDown>--%>
<h1>Live Event Search Engine</h1><br />
State: <asp:DropDownList ID="ddlState" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1" DataTextField="ST_Code" DataValueField="ST_Code" /><asp:SqlDataSource
    ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:dbConnection %>"
    SelectCommand="SELECT [ST_Code] FROM [State]">
    </asp:SqlDataSource>
City:  <asp:DropDownList ID="ddlCity" runat="server" DataSourceID="SqlDataSource2" DataTextField="RS_City" DataValueField="RS_City" /><asp:SqlDataSource
    ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:dbConnection %>"
    SelectCommand="web_PublicProgramListbyState" SelectCommandType="StoredProcedure">
<SelectParameters>
        <asp:SessionParameter Name="State" SessionField="ST_Code" Type="String" />
    </SelectParameters>
</asp:SqlDataSource>
       <asp:Button ID="Submit" runat="server" Text="Submit" />

</asp:Content>

and the code behind is

Public Sub ddlState_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ddlState.SelectedIndexChanged
        Me.ddlCity.Items.Clear()
        Dim da As New SqlDataAdapter("web_PublicProgramListbyState", New SqlConnection(ConfigurationManager.AppSettings("dbConnection")))
        Dim ds As New DataSet
        da.Fill(ds, "@State")
        da.Dispose()
        Me.ddlCity.DataSource = ds.Tables("Products")
        Me.ddlCity.DataTextField = "ProductName"
        Me.ddlCity.DataValueField = "ProductID"
        Me.ddlCity.DataBind()
    End Sub
End Class

Answer 1

Looks like you are trying to bring a dataset into this when you already have a SqlDataSource defined. Just modify the parameters of the SqlDataSource and re-bind:

Public Sub ddlState_SelectedIndexChanged(...)
    SqlDataSource2.SelectParameters.Clear()
    SqlDataSource2.SelectParameters.Add(New Parameter("@State", DbType.String, ddlState.SelectedValue))
    ddlCity.DataBind()
End Sub

Edit: Or you can use a ControlParameter referencing ddlState.SelectedValue in SqlDataSource2.SelectParameters as mentioned in another answer. Only trick there is you have to manage your default values carefully so ddlCity only binds when you want it to.