Hello Mammu,

When you are using Load On Demand, the DataSource property needs to be set each and every time the server is hit. This is necessary, because when Load On Demand is used, RadListBox doesn’t keep track of its items. They are not added to the Items collection and thus not allowing manipulating them on the server.

Take a look at the following help article and demo for more on this.
Load On Demand
ListBox / Load On Demand

Thanks,
Shinu.

Hi Shinu,

Thank you for your quick reply. Here is how the situation is. I have a User Control that has a RadListBox. when users click on a button on Page then the usercontrol loads on a popup and at the click event I want to give the listbox  datasource and call databind it. In that case how can I put it on Page_Load event. Any suggestions are appreciated.

Thank You!!!

Hello Mammu,

Did you set the Height property of the RadListBox?
You may try to bind it on the Page_Load event of the user control.

Regards,
Helen
the Telerik team