ASP.NET mvc Ajax Helper DropDownListFor send selected item value as parameter to controller

Problem I want my Ajax form to pass the value of the selected DropDownListFor to the controller but I can't figure out why the controller is not taking any value. I am working with ASP.NET MVC and I would like to stick with the helper functions as much as I can.

View

@using (Ajax.BeginForm(new AjaxOptions {
HttpMethod = "Get", 
UpdateTargetId = "UserResults",
InsertionMode = System.Web.Mvc.Ajax.InsertionMode.Replace }))
{
    @Html.DropDownListFor(Model => Model.Roles, new SelectLi(ViewBag.Groups 
    as System.Collections.IEnumerable, "Value", "Text"), "Select a group",
    new { id = "Value", onchange = "$(this.form).submit();" })
}
@Html.Partial("_UsersGroup", ViewData)

Controller

public ActionResult test(int? selectGroup)
{

// Generate dropdownlist Groups
List<SelectListItem> groupList = new List<SelectListItem>();
var query = from s in db.Roles select s;
if (query.Count() > 0)
{
    foreach (var v in query)
    {
        groupList.Add(new SelectListItem { Text = v.Name, Value =
        v.ID.ToString() });
    }
}
ViewBag.Groups = groupList;
// End

// This part is supposed to take the passed value as parameter
if (selectGroup == null)
{
    // To do code comes here, which takes selectGroup as parameter
}

Details

The form should pass a value based on the selection to the controller which takes it as "selectGroup".

ps. this is my first time asking a question, I'm sorry if I made mistakes.

The parameter of you method needs to match the name of the control which is name="Roles" so the method should be

public ActionResult test(int? roles)

Other potential issues with your code

Your controller generates List<SelectListItem> for use by the dropdownlist. There is no need for the unnecessary extra overhead of then creating a new SelectList (which is IEnumerable<SelectListItem>) from it. The view code can simply be @Html.DropDownListFor(m => m.Roles, (IEnumerable<SelectListItem>)ViewBag.Groups, "Select a group")

Do not use Model (capital M) in the expression. If you make any other reference to the model in your view (e.g. @Model.SomeProperty) you will get an error. Using lower case model => model.somProperty is OK but you can simply use m => m.someProperty

The helper will generate an id attribute (in you case id="Role") so it seems unclear why you are adding new { id = "Value", ..}, especially since you don't appear to be referencing the element by its id anywhere

Learn to use Unobtrusive Javascript rather than polluting you mark up with behavior. Remove the onclick attribute and use $('#Roles').change(function() { $('form').submit(); });