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;              
}