spero che ci sia ancora qualcuno che guarda questo thread...
non venendone fuori ho ridotto tutto all'osso ho fatto un test molto semplice

ASPX
codice:
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="test1.aspx.vb" Inherits="FrontEndASCX_test_test1" %>
 <%@ Register TagPrefix="UC" TagName="UCControl" Src="~/FrontEndASCX/test/test.ascx" %>  
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml"> 
<head runat="server">     
<title></title> </head>
 <body>   
  <form id="form1" runat="server">  
   <div>     
      <asp:ScriptManager ID="ScriptManager1" runat="server">         </asp:ScriptManager> 
             <telerik:radajaxmanager ID="RadAjaxManager1" runat="server"> 
            <AjaxSettings>        

         <telerik:AjaxSetting AjaxControlID="RadAjaxManager1">  
                   <UpdatedControls>       
          
        <telerik:AjaxUpdatedControl ControlID="DropDownList1" />     
                </UpdatedControls>     
  
          </telerik:AjaxSetting>      
       </AjaxSettings>   
      </telerik:radajaxmanager>         
                        <asp:DropDownList ID="DropDownList1" runat="server">   
              </asp:DropDownList>   
                        <asp:UpdatePanel ID="myPanel" runat="server" UpdateMode="Conditional">     
        <ContentTemplate>          
        <UC:UCControl id="UCControl1" runat="server" />    
              </ContentTemplate>  
       </asp:UpdatePanel>    
          </div>  
     </form> 
</body>
 </html>
ASPX.vb
codice:
Partial Class FrontEndASCX_test_test1     Inherits System.Web.UI.Page   
    Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init    
     fillCombo()    
 End Sub      
Private Sub fillCombo()       
  Dim a As New List(Of Long)     
    a.Add(1)     
    a.Add(2)      
   a.Add(3)     
    a.Add(4)        
  DropDownList1.AutoPostBack = True     
    DropDownList1.DataSource = a   
      DropDownList1.DataBind()    
 End Sub    

  Protected Sub DropDownList1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownList1.TextChanged    
     'UCControl1.IDCart = 1     
    UCControl1.setCart(DropDownList1.SelectedValue)  
   End Sub End Class
ASCX


codice:
<%@ Control Language="VB" AutoEventWireup="false" CodeFile="test.ascx.vb" Inherits="FrontEndASCX_test_test" %> 
<asp:Label ID="Label1" runat="server" Text="__"></asp:Label>
ASCX.vb
codice:
 Partial Class FrontEndASCX_test_test     Inherits System.Web.UI.UserControl  
    Dim myidcart As Long 
     Public Property IDCart() As Long    
     Get             Return myIDCart      
   End Get       
  Set(ByVal value As Long)           
  myidcart = value     
    End Set  
   End Property     

 Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init        
 Label1.Text += "Page_Init " & IDCart   
  End Sub   
   Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load  
       Label1.Text += "Page_Load " & IDCart   
  End Sub     
 Public Sub setCart(ByVal parCart As Long)     
    IDCart = parCart        
 Label1.Text += "setCart " & parCart    
 End Sub
  End Class
come metto ajaxato il controllo combo (nel mio caso reale è una radgrid ma credo che il test vada bene uguale) non funziona più niente.
cosa avrei dovuto scrivere per farlo funzionare con ajax??
Grazie