Error 500 - CascadingDropDown AJAX Extender toolkit control

I am trying to populate 3 dropdown lists with using CascadingDropDown. DDLs shows make, model and color of a car respectively. Data resides in a .xml file.

The last DDL is having AutoPostBack = true and I am trying to display the values from dropdown in one label control.

I am pulling data from a webservice to populate DDLs. I am not getting any error while compiling but when I run the code it shows Error 500. I am not sure where I am going wrong. I am following the tutorial video of 'How Do I' -

http://www.asp.net/ajax/videos/how-do-i-use-the-aspnet-ajax-cascadingdropdown-control-extender

I am using C# and .Net framework 2.0.

My Web Service looks like this -

[WebService(Namespace = "http://tempuri.org/")] [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)] [System.Web.Script.Services.ScriptService()] public class CarsService : System.Web.Services.WebService {

public XmlDataDocument _document;
//public object _lock = new Object();
public CarsService () {

    //Uncomment the following line if using designed components 
    //InitializeComponent(); 
}

public XmlDocument Document
 {
     get
     {
         if (_document==null)
         {
             _document = new XmlDataDocument();
             _document.Load(HttpContext.Current.Server.MapPath("~/App_Data/CarsService.xml"));
         }
         //Document = _document;
         return _docume开发者_StackOverflow中文版nt;
     }
 }

public string[] Hierarchy
{
    get
    {
        string[] hierarchy = {"make","model"};
        return hierarchy;
    }

}

[WebMethod]
public AjaxControlToolkit.CascadingDropDownNameValue[] GetDropDownContents(string knowncategoryvalues, string category)
{
    StringDictionary knownCategoryValuesDictionary = new StringDictionary();
    knownCategoryValuesDictionary = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knowncategoryvalues);

    return  AjaxControlToolkit.CascadingDropDown.QuerySimpleCascadingDropDownDocument(Document, Hierarchy, knownCategoryValuesDictionary, category);

}

}

And Web page looks like this -

Untitled Page

    <br />
    <br />

    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:DropDownList ID="DropDownList1" runat="server" Width="147px">
            </asp:DropDownList>
            <br />
            <asp:DropDownList ID="DropDownList2" runat="server" Width="147px">
            </asp:DropDownList>
            <br />
            <asp:DropDownList ID="DropDownList3" runat="server" Width="147px" 
                AutoPostBack="True">
            </asp:DropDownList>
            <br />
            <br />
            <br />
            <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
            <br />
            <br />
            <asp:TextBox ID="TextBox1" runat="server" Height="64px" Width="191px"></asp:TextBox>
            <br />
            <br />
            <ajaxToolkit:CascadingDropDown ID="CascadingDropDown1" runat="server" 
                Category="make" PromptText="Please enter dish name" 
                ServiceMethod="GetDropDownContents" TargetControlID="DropDownList1" 
                ServicePath="CarsService.asmx">
            </ajaxToolkit:CascadingDropDown>
            <ajaxToolkit:CascadingDropDown ID="CascadingDropDown2" runat="server" 
                Category="model" LoadingText="[Loading price...]" 
                ParentControlID="DropDownList1" PromptText="Please select price" 
                ServiceMethod="GetDropDownContents" TargetControlID="DropDownList2">
            </ajaxToolkit:CascadingDropDown>
            <ajaxToolkit:CascadingDropDown ID="CascadingDropDown3" runat="server" 
                Category="color" LoadingText="[Loading Description...]" 
                ParentControlID="DropDownList2" PromptText="Select Description" 
                ServiceMethod="GetDropDownContents" TargetControlID="DropDownList3">
            </ajaxToolkit:CascadingDropDown>
            <br />
            <br />
            <br />
            <br />
        </ContentTemplate>
    </asp:UpdatePanel>
</form>

Attaching xml file also here -

I am not sure where I am going wrong. Can you please help me ?

Firebug is invaluable for debugging web service calls.

1. Install the Firebug plugin for Firefox
2. Open your test web page in Firefox
3. Open Firebug, then open the Net tab and click on XHR
4. Try your drop down again and look for the error in the XHR response