I was working with the AJAX MultiHandleSliderExtender control, the thing that bother me the most is the error listed below at design time. The code didn't throw any error at runtime.
Error Creating Control - MultiHandleSliderExtender1
'cc1:MultiHandleSliderExtender could not be set on property MultiHandleSliderTargets'
Here is the markup code
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<cc1:MultiHandleSliderExtender ID="MultiHandleSliderExtender2" runat="server">
<cc1:MultiHandleSliderTarget ControlID="someID" />
After looking into the source code carefully, I noticed that the MultiHandleSliderTargets control is missing a TagPrefix next to it. Placing the TagPrefix "cc1:MultiHandleSliderTargets" has solved the problem.