UpdatePanel DropDownList.SelectedIndexChanged event

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C#

ASP.NET

Ajax

I have used a DropDownList in UpdatePanel and i have handled SelectedIndexChanged and it is firing successfully, but in that i unable to get Correct SelectedValue, why..?
Here is my code

<asp:UpdatePanel ID="NetworkUpadated" UpdateMode="Conditional" runat="server">
<ContentTemplate>
    NMI: <asp:TextBox ID="NMITextBox"  OnTextChanged="GetNetwork" AutoPostBack="true" runat="server"></asp:TextBox>

    Network Code:   <asp:DropDownList ID="NetworkCodeDropDown" AutoPostBack="true" OnSelectedIndexChanged="NetworkCodeDropDown_Changed" runat="server" > </asp:DropDownList>
  </ContentTemplate>
</asp:UpdatePanel>

and in my code behind

public void NetworkCodeDropDown_Changed(object sender, EventArgs e)
       {
           string sel = NetworkCodeDropDown.SelectedValue;// it showing always 1
       }

If i check the request in Fidler it is going correct selected value,
but why i unable to get my selected value in code behind..
please help me