Set updatepanelanimationextender TargetControId at codebehind - asp.net

Question

Inside usercontrol I have updatepanelanimationextender, when I add this control to a webpage I want to pass updatepanel's Id as parameter to control's property.
Usercontrol:
public partial class Controls_UpdateProgress : System.Web.UI.UserControl
{
public string UpdatePanelID { get; set; }
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
UpdatePanelAnimationExtender1.TargetControlID = UpdatePanelID;
}
}
}
<cc1:updatepanelanimationextender id="UpdatePanelAnimationExtender1" runat="server" >
<Animations>
<OnUpdating>
<Parallel duration="0" >
<ScriptAction Script="onUpdating();" />
</Parallel>
</OnUpdating>
<OnUpdated>
<Parallel duration="0">
<ScriptAction Script="onUpdated();" />
</Parallel>
</OnUpdated>
</Animations>
</cc1:updatepanelanimationextender>
WebPage: UpdatePanel1 is an id of the updatepanel.
<uc1:UpdateProgress ID="UpdateProgress1" runat="server" UpdatePanelID="UpdatePanel1" />
I get error:
The TargetControlID of
'UpdatePanelAnimationExtender1' is not
valid. The value cannot be null or
empty.

Answer 1

The AJAX control extenders do not store their TargetControlID in the ViewState (I checked in System.Web.Extensions 3.5 with Reflector and that property only gets/sets a private member). Thus, the value you stored is lost on postback.
You'll have to store the value in every request:
protected void Page_Load(object sender, EventArgs e)
{
UpdatePanelAnimationExtender1.TargetControlID = UpdatePanelID;
}

Answer 2

To avoid that exception, you should first make sure that UpdatePanelID is not null...
if (!IsPostBack) {
if (UpdatePanelID != Null) {
UpdatePanelAnimationExtender1.TargetControlID = UpdatePanelID;
}
}
If you desire to set the property of the UpdatePanelID from the parent page programmatically you will need to cast UpdateProgress1 as a Controls_UpdateProgress instance. To do that, do something like this...
((Controls_UpdateProgress)UpdateProgress1).UpdatePanelID = "ThisIsTheIdYouWishToSet";