I'm attempting to get the cascadingdropdown to work and get this weird error. I've followed the cascadingdropdown database walk-through thoroughly. It's extremely frustrating to see these great controls that are such a pain in the butt to use. I'm using and XP machine with Visual Studio 2005 and I'm only using the local ASP.net development server to test the page.

Do I need to import the ExtenderControlBase.cs into my website? There is no mention of this in the walk through directions or environment setup.

Here is the stack trace from the error.

at AjaxControlToolkit.CascadingDropDown.CascadingDropDown_ClientStateValuesLoaded(Object sender, EventArgs e) in d:\TFS\AtlasControlToolkit\Release\AjaxControlToolkit\CascadingDropDown\CascadingDropDownExtender.cs:line 188
   at AjaxControlToolkit.ExtenderControlBase.LoadClientStateValues() in d:\TFS\AtlasControlToolkit\Release\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs:line 386
   at AjaxControlToolkit.ExtenderControlBase.Page_PreLoad(Object sender, EventArgs e) in d:\TFS\AtlasControlToolkit\Release\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs:line 350
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Web.UI.Page.OnPreLoad(EventArgs e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

 Here is the dissembly: 

--- d:\TFS\AtlasControlToolkit\Release\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs

I just ran into the same problem and I figured I would answer in case someone else is having this issue.

 The name of this extender is missleading. It's called CascadingDropDown when it should be called something like CascadingDropDownExtender.

The extender DOES NOT output a dropdownlist. It populates an existing one. If you are like me, you probably don't have a TargetControlID set for the extender, only a ParentControlID. Add a combo box and set the extender's TargetControlID property to the new combo ID and you should be good to go.