使用ASP.NET过滤器参数时如何添加空白列表项

Question

A default list item has been added to each dropdown.

When an option is selected from ddlA the items in ddlB will be filtered.

PROBLEM

When using the ASP.NET filter parameter the default list item value=0 will be removed/filtered out from ddlB .

QUESTION

How can the default list item be kept or re-added to ddlB when the list is filtered?

---

DROPDOWNLIST

<asp:DropDownList ID="ddlA" runat="Server" DataSourceID="sqlA" DataTextField="TextA" DataValueField="ValueA" AppendDataBoundItems="true" AutoPostBack="true">
  <asp:ListItem Text="Select ..." Value="0"/>
</asp:DropDownList>

<asp:DropDownList ID="ddlB" runat="Server" DataSourceID="sqlB" DataTextField="TextB" DataValueField="ValueB" AppendDataBoundItems="true">
  <asp:ListItem Text="Select ..." Value="0"/>
</asp:DropDownList>

DATASOURCE

<asp:SqlDataSource ID="sqlA" runat="Server" SelectCommand="spA" SelectCommandType="StoredProcedure"/>

<asp:SqlDataSource ID="sqlB" runat="Server" SelectCommand="spB" SelectCommandType="StoredProcedure" FilterExpression="ValueA= '{0}'">
  <FilterParameters>
    <asp:ControlParameter Name="Col" ControlID="ddlA" PropertyName="SelectedValue"/>
  </FilterParameters>
</asp:SqlDataSource>

Answer 1

you will need to have this initial value in your data source so when creating datasource you will need to

SELECT 'Select' as TextA, 0 as ValueA
JOIN
Select TextA, ValueA from yourTable

these values will be bound to your DropDownLists and stay there and one more thing. Remember to remove filter before refiltering

SqlDataSource1.FilterExpression=""