Different values in dropdownlist depends on what i select in another dropdownlist

Question

What I want to make and Create: I want to make 2 dropdownlist in asp.net. Lets say the first contains countries, and the second contains the cities in the countries.

Lets say I select England in the first Dropdownlist and then only the Cities that belongs to England will be visible or selectable in the second dropdownlist.

<asp:DropDownList ID="Countries" runat="server">  // First dropdownlist
    <asp:ListItem>England</asp:ListItem>
    <asp:ListItem>Denmark</asp:ListItem>
</asp:DropDownList>

<asp:DropDownList ID="Citys" runat="server">    // Second dropdownlist
</asp:DropDownList>

I hope I explained it good enough and I hope that someone can provide me with either some code that can do it or a hint about it.

Answer 1

If you use SqlDataSource, you can cascade the selected value to CitySqlDataSource.

Here the demo -

Database

   Countries                                Cities
Id       Name                    Id      CountryId      Name
1        England                 1            1         London
2        Denmark                 2            2         Copenhagen    

ASPX

<asp:DropDownList ID="CountryDropDownList" runat="server"
    DataSourceID="CountrySqlDataSource" 
   DataTextField="Name" DataValueField="Id" AutoPostBack="True">
</asp:DropDownList>
<asp:SqlDataSource ID="CountrySqlDataSource" runat="server"
    ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
    SelectCommand="SELECT Id,Name FROM [Countries]"></asp:SqlDataSource>

<asp:DropDownList ID="CityDropDownList" runat="server"
    DataSourceID="CitySqlDataSource" 
    DataTextField="Name" DataValueField="Id">
</asp:DropDownList>
<asp:SqlDataSource ID="CitySqlDataSource" runat="server"
    ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
    SelectCommand="SELECT Id,Name FROM Cities WHERE CountryId=@CountryId">
    <SelectParameters>
        <asp:ControlParameter ControlID="CountryDropDownList" 
            PropertyName="SelectedValue"
            Name="CountryId " Type="String" 
            DefaultValue="England" />
    </SelectParameters>
</asp:SqlDataSource>

Here is the similar answer.