下拉列表 MVC 4 错误

Question

我试图让一个下拉列表工作，但它不适合我。此应用程序主要是基于节日的应用程序，您可以在其中添加节日以及您的活动。我得到的错误在线：

@Html.DropDownList("towns", (IEnumerable<SelectListItem>)ViewData["Town"], new{@class = "form-control", @style="width:250px" })

这是我得到的错误：

没有具有键“城镇”的“IEnumerable”类型的 ViewData 项。

创建.cshtml

<div class="form-group">
@Html.LabelFor(model => model.FestivalTown, new { @class = "control-label col-md-2" })      
<div class="col-md-10">
@Html.DropDownList("towns", (IEnumerable<SelectListItem>)ViewData["Town"], new{@class = "form-control", @style="width:250px" })
@Html.ValidationMessageFor(model => model.FestivalTown)
</div>
@*@Html.Partial("ddlFestivalCounty");*@
</div>

Controller.cshtml

//Get
List<SelectListItem> Towns = new List<SelectListItem>();
Towns.Add(new SelectListItem { Text = "Please select your Town", Value = "SelectTown" });
var towns = (from t in db.Towns select t).ToArray();
for (int i = 0; i < towns.Length; i++)
{
Towns.Add(new SelectListItem
{
Text = towns[i].Name,
Value = towns[i].Name.ToString(),
Selected = (towns[i].ID == 0)
});
}

ViewData["Town"] = Towns;

//Post
festival.FestivalTown.Town = collection["Town"];

模型.cs

    public class Festival
{
    public int FestivalId { get; set; }

    [Required]
    [Display(Name = "Festival Name"), StringLength(100)]
    public string FestivalName { get; set; }

    [Required]
    [Display(Name = "Start Date"), DataType(DataType.Date)]
    public DateTime StartDate { get; set; }

    [Required]
    [Display(Name = "End Date"), DataType(DataType.Date)]
    public DateTime EndDate { get; set; }

    [Required]
    [Display(Name = "County")]
    public virtual County FestivalCounty { get; set; }

    [Display(Name = "Festival Location")]
    public DbGeography Location { get; set; }

    [Required]
    [Display(Name = "Town")]
    public virtual Town FestivalTown { get; set; }

    [Required]
    [Display(Name = "Festival Type")]
    public virtual FestivalType FType { get; set; }

    public UserProfile UserId { get; set; }
}

    public class Town
{
    public int ID { get; set; }
    [Display(Name = "Town")]
    public string Name { get; set; }
}

Answer 1

我怀疑当您将表单提交到[HttpPost] 操作时会发生此错误，而不是在呈现表单时发生，对吧？这个动作会渲染包含下拉菜单的同一个视图，对吧？在这个[HttpPost] 操作中，您忘记填充ViewData["Town"] 值，就像您在HttpGet 操作中所做的那样，对吧？

所以，继续以与 GET 操作相同的方式填充此属性。当您将表单提交到 [HttpPost] 操作时，仅选定的值会发送到控制器。因此，如果您打算重新显示相同的视图，则需要重新填充集合值，因为此视图会呈现一个下拉列表，该下拉列表试图从 ViewData["Town"] 绑定其值。

这就是我在代码方面的意思：

[HttpPost]
public ActionResult SomeAction(Festival model)
{
    ... bla bla bla

    // don't forget to repopulate the ViewData["Town"] value the same way you did in your GET action
    // if you intend to redisplay the same view, otherwise the dropdown has no way of getting
    // its values
    ViewData["Town"] = ... same stuff as in your GET action

    return View(model);
}

话虽如此，我强烈建议您使用视图模型而不是这种 ViewData/ViewBag 弱类型的东西。不仅您的代码会变得更加干净，而且甚至错误消息也会开始变得有意义。