ASP.NET Gridview filtering with Dropdowns - asp.net

Question

I have a Gridview which uses a dropdown list to provide a value applied to a SQL query which filters to the applicable data. The issue is that when an item is selected from the list the gridview does not update, the DefaultValue remains as the filter.
I've tried a number of alternatives seen though StackOverflow like by directly binding the value in the markup but to no success yet.
<label for="City" class="control-label">Filter by City:</label>
<div>
<asp:DropDownList ID="CityDropdown" runat="server" Width="123px" >
<asp:ListItem>Aberdeen</asp:ListItem>
<asp:ListItem>Armagh</asp:ListItem>
<asp:ListItem>Bangor</asp:ListItem>
<asp:ListItem>Bath</asp:ListItem>
</asp:DropDownList>
</div>
</div>
<hr />
<asp:Panel ID="PanelFoodbanks" runat="server">
<asp:GridView ID="ListFoodbanks" runat="server" AutoGenerateColumns="False" AllowSorting="True" DataSourceID="SqlDataSource_FindCity" EmptyDataText="There are no data records to display." CssClass="table table-responsive" GridLines="None">
<Columns>
<asp:BoundField DataField="Id" SortExpression="Id" ItemStyle-CssClass="hidden" HeaderStyle-CssClass="hidden">
<HeaderStyle CssClass="hidden" />
<ItemStyle CssClass="hidden" />
</asp:BoundField>
<asp:BoundField DataField="Other Field" HeaderText="OF" SortExpression="Other Field" />
<asp:BoundField DataField="City" HeaderText="City" SortExpression="City" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource_FindFoodbanks" runat="server" ConnectionString="<%$ ConnectionStrings:dbcon %>" SelectCommand="SELECT * FROM [Table] WHERE ([City] = #CityDropdown)">
<SelectParameters>
<asp:Parameter Name="CityDropdown" DefaultValue="Aberdeen"/>
</SelectParameters>
</asp:SqlDataSource>
Preferably the gridview just update dynamically when a different value in the dropdown is selected, rather than having the need to press another button etc.
Thanks in advance for any help or pointers.

Answer 1

I believe you need to use ControlParameter bound to city dropdown list control in your SQL Datasource:
<asp:SqlDataSource ID="SqlDataSource_FindFoodbanks" runat="server" ConnectionString="<%$ ConnectionStrings:dbcon %>" SelectCommand="SELECT * FROM [Table] WHERE ([City] = #CityDropdown)">
<SelectParameters>
<asp:ControlParameter ControlID="CityDropdown" Name="CityDropdown"
PropertyName="SelectedValue" />
</SelectParameters>
</asp:SqlDataSource>
Also add AutoPostBack="true" ondropdown list
<asp:DropDownList ID="CityDropdown" AutoPostBack="true" runat="server" Width="123px" >
Note: I noticed that in your code GridView is using DataSourceID SqlDataSource_FindCity and not SqlDataSource_FindFoodbanks