user control's dropdown causing full postback inside updatepanel
<p>I am working on asp.net application and I have an update panel like this:</p>

<pre><code>   &lt;asp:UpdatePanel ID="upCheckout" runat="server"&gt;
                    &lt;ContentTemplate&gt;
                        &lt;!-- BillingAddress --&gt;
                        &lt;div runat="server" id="pnlBillingAddress" class="checkoutstep"&gt;

                            &lt;asp:Panel runat="server" ID="pnlBillingAddressContent" class="stepcontent"&gt;
                                &lt;nopCommerce:CheckoutBillingAddress ID="ctrlCheckoutBillingAddress" runat="server"
                                    OnePageCheckout="true" OnCheckoutStepChanged="ctrlCheckoutBillingAddress_CheckoutStepChanged" /&gt;


                                &lt;asp:CheckBox ID="chkShippingSameAsBilling" runat="server" Text="  Ship to same address"
                                    AutoPostBack="true" Checked="true" onclick="ShowShippingAddress();" /&gt;&lt;br /&gt;
                            &lt;/asp:Panel&gt;
                        &lt;/div&gt;
                        &lt;!-- ShippingAddress --&gt;
                        &lt;div runat="server" id="pnlShippingAddress" class="checkoutstep"&gt;                                
                            &lt;asp:Panel runat="server" ID="pnlShippingAddressContent" class="stepcontent"&gt;
                                &lt;nopCommerce:CheckoutShippingAddress ID="ctrlCheckoutShippingAddress" runat="server"
                                    OnePageCheckout="true" OnCheckoutStepChanged="ctrlCheckoutShippingAddress_CheckoutStepChanged" /&gt;                                    
                            &lt;/asp:Panel&gt;
                        &lt;/div&gt;
                    &lt;/ContentTemplate&gt;
                &lt;/asp:UpdatePanel&gt;
</code></pre>

<p>where billing address is a user control. In billing address control, There is a dropdownlost. like this:</p>

<pre><code>  &lt;asp:DropDownList ID="drpBillingAddresses" ClientIDMode="Static" runat="server" AutoPostBack="True" OnSelectedIndexChanged="drpBillingAddresses_SelectedIndexChanged"&gt;
                        &lt;/asp:DropDownList&gt;
</code></pre>

<p>but when I change dropdown selection, I get full post back instead of partial postback. why I am getting full postback ?</p>
