how to use OutputCache in a userControl in asp.net - asp.net

Question

I have a aspx page that have this piece of code to load a usercontrol loaded from database
Control userControl = new Control();
userControl = LoadControl(userControlName);
((HiddenField)userControl.FindControl("HiddenFieldCategoryID")).Value = categoryID.ToString();
((HiddenField)userControl.FindControl("HiddenFieldNewsID")).Value = newsID.ToString();
((HiddenField)userControl.FindControl("HiddenFieldTypeID")).Value = typeID.ToString();
PlaceHolder3.Controls.Add(userControl);
and the ascx have an outputcache
<%# OutputCache Duration=10 VaryByParam="none" %>
when i browse the page
this error comes out
[NullReferenceException: Object
reference not set to an instance of an
object.]
Content_SectionNews.Page_Load(Object
sender, EventArgs e) in c:\Documents
and Settings\Administrator\My
Documents\Visual Studio
2005\Projects\AnaweenNews.root\AnaweenNews\anaween
website\Content\SectionNews.aspx.cs:127
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr
fp, Object o, Object t, EventArgs e)
+14 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object
sender, EventArgs e) +35
System.Web.UI.Control.OnLoad(EventArgs
e) +99
System.Web.UI.Control.LoadRecursive()
+50 System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean
includeStagesAfterAsyncPoint) +627
Version Information: Microsoft .NET
Framework Version:2.0.50727.3615;
ASP.NET Version:2.0.50727.3618

Answer 1

The type that returned from the LoadControl it will be PartialCachingControl, please follow the steps to how use the PartialCachingControl:
PartialCachingControl userControl = LoadControl(userControlName) as PartialCachingControl;
PlaceHolder3.Controls.Add(userControl);
if(userControl.CachedControl != null)
{
((HiddenField)userControl.CachedControl.FindControl("HiddenFieldCategoryID")).Value = categoryID.ToString();
((HiddenField)userControl.CachedControl.FindControl("HiddenFieldNewsID")).Value = newsID.ToString();
((HiddenField)userControl.CachedControl.FindControl("HiddenFieldTypeID")).Value = typeID.ToString();
}