Donut caching and Substitution control

Hi,
I want to use donut caching with the Substitution control. I created a DropDownList control and i put it in ddllanguage.ascx
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<%@ Control Language="VB" AutoEventWireup="false" CodeFile="DDLLanguage.ascx.vb"
    Inherits="controls_DDLLanguage" %>
<asp:DropDownList ID="DDLLang" runat="server" AutoPostBack="True" ViewStateMode="Enabled">
    <asp:ListItem Value="en-US">English</asp:ListItem>
    <asp:ListItem Value="fr-CA">Français</asp:ListItem>
</asp:DropDownList>
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I put a Substitution control on my page
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<asp:Substitution ID="Substitution1" runat="server" MethodName="DropDown" />
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And now i would like ...