Get Value After Postback Time

<asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
      <Triggers>
        <asp:AsyncPostBackTrigger ControlID="DropDownList1" EventName ="DropDownList1_SelectedIndexChanged" />
        <%--<asp:PostBackTrigger ControlID ="DropDownList1" />--%>
        </Triggers>
        <ContentTemplate>
           <table class="style1">
                <tr>
                    <td>
                        <asp:FileUpload ID="FileUpload1" runat="server" />
                    </td>
                    <td>
                        &nbsp;
                    </td>
                    <td>
                        &nbsp;
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                            <asp:ListItem>Select</asp:ListItem>
                            <asp:ListItem>1</asp:ListItem>
                            <asp:ListItem>2</asp:ListItem>
                        </asp:DropDownList>
                    </td>
                    <td>
                        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        &nbsp;
                    </td>
                </tr>
                <tr>
                    <td>
                        <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
                    </td>
                    <td>
                        &nbsp;
                    </td>
                    <td>
                        &nbsp;
                    </td>
                </tr>
            </table>
        </ContentTemplate>

    </asp:UpdatePanel>