in ASp.NET MVC, How do you remember a dropdownlists state?

Question

I have this action:

public ActionResult Add([Bind(Include = "Title,Description")] product product2Add){ --SNIP-- }

I'm using a view model pattern where I've created a special "AddProduct_ViewModel" class that contains my product class and anything else I need in my view. That includes 2 dropdownlists; one that does a DB lookup of all the various product catagories, and another that lists the product suppliers.

When I do validation on my product that fails I cannot see how to easily save the state of my dropdownlists (without doing some lengthy db code). Is there a better way?

Answer 1

Well.. in MVC there is no viewstate, so controls can't save state, although you can use some tricks with storing it in a cookie using javascript - preferably jquery, or saving this information to the server side in onChange event.

Answer 2

in the controller you can call:

Request.Form.Get("MyDropDownListId");

To get the selected option's value attribute.

While building up the dropdownlist again after postback, you can use this value to re-set the selected item.

Example of setting the selected item in the view when you have a property "PageSize" in your Model:

Response.Write(Html.DropDownList("pageSize",
            Model.PageSizes.ToSelectList(p => p.ToString(), p => p.ToString(), p => p.Equals(Model.PageSize))));

With extension method ToSelectList():

public static List<SelectListItem> ToSelectList<T>(this IEnumerable<T> enumerable, Func<T, string> text, Func<T, string> value, Func<T, bool> selected)
{
    var items = enumerable.Select(f => new SelectListItem() { Text = text(f), Value = value(f) }).ToList();
    return items;
}

Answer 3

The best way i have found is like this.

viewmodel:

public class ViewModel
{
    public int? DropdownValue { get; set; }
}

view:

<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage<ViewModel>" %>
<%= Html.DropDownList("DropdownValue") %>

controller:

public ActionResult Edit()
{
    ViewData["DropdownValue"] = CreateListOfSelectListItem();
    View(GetViewModelFromDb());
}

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(ViewModel postedData)
{
    ViewData["DropdownValue"] = CreateListOfSelectListItem();
    UpdateDb(postedData);
    View();
}

This method keeps your viewmodel clean of the select options, and uses the model state so you can easily add validation. If you an option without a value, you can just validate on not null.

Hope it helps.