下拉列表MVC 4錯誤

Question

我試圖讓下拉列表工作，但它不適合我。此應用程序主要是一個基於節日的應用程序，您可以在活動中添加節日。我得到的錯誤是在線：

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

這是我的錯誤：

有型「的IEnumerable」具有關鍵的「城鎮」的無ViewData的項目。

Create.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"]; 

Model.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

我懷疑，出現此錯誤當您提交的形式向[HttpPost]行動，不是當你渲染表單時，對吧？這個動作呈現包含下拉列表的相同視圖，對吧？在這個[HttpPost]動作中，你忘記了像在你的HttpGet動作中那樣填充ViewData["Town"]值，對吧？

因此，請繼續並按照您在GET操作中所做的相同方式填充此屬性。當您將表單提交到您的[HttpPost]動作時，只有將選定的值發送到控制器。因此，如果您打算重新顯示相同視圖，則需要重新填充集合值，因爲此視圖會呈現一個下拉列表，它試圖綁定來自ViewData["Town"]的值。

這裏就是我的意思在代碼方面：

[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); 
} 

而這一切都這樣說，我會多強烈建議您使用視圖模型，而不是這個的ViewData/ViewBag弱類型的東西。不僅如此，您的代碼將變得更加乾淨，但即使錯誤消息也將開始有意義。