validating cascading dropdownlist

i am working on MVC.Net. in that i have used cascading dropdownlist. I want to do validations for blank field.

the view page coding is:

Select Category: <%= Html.DropDownList("Makes", ViewData["Makes"] as SelectList,"Select Category")%> Select Subcategory: <%= Html.CascadingDropDownList("Models", "Makes")%>

the code on controller:

public ActionResult AddSubCategoryPage() {

         var makeList = new SelectList(entityObj.Category.ToList(), "Category_id", "Category_name");
         ViewData["Makes"] = makeList;

         // Create Models view data
         var modelList = new CascadingSelectList(entityObj.Subcategory1.ToList(), "Category_id", "Subcategory_id", "Subcategory_name");
         ViewData["Models"] = modelList;

         return View("AddSubCategoryPage");
     }

and for that i have made one class:

public static class JavaScriptExtensions { public static string CascadingDropDownList(this HtmlHelper helper, string name, string associatedDropDownList) { var sb = new StringBuilder();

        // render select tag
        sb.AppendFormat("<select name='{0}' id='{0}'></select>", name);
        sb.AppendLine();

        // render data array
        sb.AppendLine("<script type='text/javascript'>");
        var data = (CascadingSelectList)helper.ViewDataContainer.ViewData[name];
        var listItems = data.GetListItems();
        var colArray = new List<string>();
        foreach (var item in listItems)
            colArray.Add(String.Format("{{key:'{0}',value:'{1}',text:'{2}'}}", item.Key, item.Value, item.Text));
        var jsArray = String.Join(",", colArray.ToArray());
        sb.AppendFormat("$get('{0}').allOptions=[{1}];", name, jsArray);
        sb.AppendLine();
        sb.AppendFormat("$addHandler($get('{0}'), 'change', Function.createCallback(bindDropDownList, $get('{1}')));", associatedDropDownList, name);
        sb.AppendLine();
        sb.AppendLine("</script>");

        return sb.ToString();

    }
}

public class CascadingSelectList
{
    private IEnumerable _items;
    private string _dataKeyField;
    private string _dataValueField;
    private string _dataTextField;

    public CascadingSelectList(IEnumerable items, string dataKeyField, string dataValueField, string dataTextField)
    {
        _items = items;
        _dataKeyField = dataKeyField;
        _dataValueField = dataValueField;
        _dataTextField = dataTextField;
    }

    public List<CascadingListItem> GetListItems()
    {
        var listItems = new List<CascadingListItem>();
        foreach (var item in _items)
        {
            var key = DataBinder.GetPropertyValue(item, _dataKeyField).ToString();
            var value = DataBinder.GetPropertyValue(item, _dataValueField).ToString();
            var text = DataBinder.GetPropertyValue(item, _dataTextField).ToString();
            listItems.Add(new CascadingListItem(key, value, text));
        }
        return listItems;
    }
}

public class CascadingListItem
{
    public CascadingListItem(string key, string value, string text)
    {
        this.Key = key;
        this.Value = value;
        this.Text = text;
    }

    public string Key { get; set; }
    public string Value { get; set; }
    public string Text { get; set; }
}

but when i run the aaplication it gives me following error:

Server Error in '/' Application. The parameters dictionary contains a null entry for parameter 'Models' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult AddSubCategoryPage(Int32, System.String, System.String)' in 'CMS.Controllers.HomeController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter. Parameter name: parameters .

plz help me.

---

The Models of type CascadingSelectList is null. Make sure an instance of the object exists and properties have no nulls. Also I would recommend using JSON to build your drop downs as bundling the combo arrays to return can create a performance hit and is confusing.

<script type="text/javascript">
    $(function() {
        $.getJSON("/Home/Make/List", function(data) {
            var items = "<option>---------------------</option>";
            $.each(data, function(i, make) {
                items += "<option value='" + make.Value + "'>" + make.Text + "</option>";
            });
            $("#Make").html(items);
        });

        $("#Make").change(function() {
            $.getJSON("/Home/Models/List/" + $("#Make > option:selected").attr("value"), function(data) {
                var items = "<option>---------------------</option>";
                $.each(data, function(i, model) {
                    items += "<option value='" + model.Value + "'>" + model.Text + "</option>";
                });
                $("#Model").html(items);
            });
        });
    });
</script>

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