Can't get value for DropDownList

Question

I'm trying to get value of DropDownList in the OnPreInit event, here's the code for it:

 

       protected override void OnPreInit(EventArgs e)

       {

       if(Request.Form["DropDownList1"]!=null)

       {

       var theme = Request.Form["DropDownList1"];

       Page.Theme = theme;

       }

       base.OnPreInit(e);

       }

 But on debugging it returns null every time, here's the code for the dropdownlist:

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">

      <div class="about-content" style="height:600px">

      <div>

      <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">

      <asp:ListItem>Theme1</asp:ListItem>

      <asp:ListItem>Theme2</asp:ListItem>

      </asp:DropDownList>

      </div>

      </asp:Content> 

 

 Also I'm doing this inside a masterpages contentplaceholder, if that helps.