ASP.Net MVC DropdownList and selected value

问题:

I was able to create a dropdownlist for my MVC project, but the value is not selected as selected value on the dropdownlist. It just shows whole county list from the first.The value is from database. I tried find it from previous post, but it was quite confusing. Please, suggest me for any idea.

Controller Code

public ActionResult Edit(int i)
{
     var items = new SelectList(db.MST_COUNTRies.ToList(), "COUNTRY_ID", "COUNTRY_NM");
     ViewData["MST_COUNTRies"] = items;
}

View Code

<%= Html.DropDownList("COUNTRY_ID", (SelectList)ViewData["MST_COUNTRies"])%>

回答1:

I would suggest you using a strongly typed views (no need of ViewData and magic strings). As always start by defining your view model class:

public class CountriesViewModel
{
    public int? SelectedCountryId { get; set; }
    public IEnumerable<SelectListItem> Countries { get; set; }
}

Then your controller:

public class HomeController : Controller
{
    public ActionResult Index(int? id)
    {
        // TODO: Fetch those from your DB
        var countries = new[] 
        {
            new { Id = 1, Name = "Country 1" },
            new { Id = 2, Name = "Country 2" },
        };

        var model = new CountriesViewModel
        {
            Countries = new SelectList(countries, "Id", "Name", id)
        };

        return View(model);
    }
}

And finally your strongly typed view:

<%@ Page Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<SomeNs.Models.CountriesViewModel>" %>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <%= Html.DropDownListFor(
            x => x.SelectedCountryId, 
            Model.Countries, 
            "-- Select Country --") 
    %>

</asp:Content>

Now when you call /home/index you get no country selected, but when you pass the id to this action like /home/index/2 you get the country with this id selected.

Remark: If you are using ASP.NET MVC 1.0 you won't have the strongly typed DropDownListFor helper and you could use this instead:

<%= Html.DropDownList(
        "SelectedCountryId", 
        Model.Countries, 
        "-- Select Country --") 
%>

回答2:

Try something like this:

public ActionResult Edit(int i)
{
     var items = new SelectList(db.MST_COUNTRies.ToList(), "COUNTRY_ID", "COUNTRY_NM", yourSelectedCountryId);
     ViewData["COUNTRY_ID"] = items;
}

Then in your view:

<%= Html.DropDownList("COUNTRY_ID"); %>

Some notes:

• The "yourSelectedCountryId" argument in new SelectList() can be used to determine the item that should be initially selected. Assuming that COUNTRY_ID is an int, this argument should also be an int.
• If you populate ViewData with an IEnumerable that matches the name/id of the DropDownList, the Html.DropDownList() call automatically finds it and uses it. That's why I changed ViewData["MST_COUNTRies"] to ViewData["COUNTRY_ID"]. It saves a little code, and I found myself forced to use this approach to work around a bug in MVC v1.