MVC的DropDownList的SelectedValue不正確顯示

Question

我試圖尋找，並沒有發現任何固定我的問題。我在Razor視圖上有一個DropDownList，它不會顯示我在SelectList中標記爲Selected的項目。下面是填充列表的控制器代碼：

var statuses = new SelectList(db.OrderStatuses, "ID", "Name", order.Status.ID.ToString()); 
ViewBag.Statuses = statuses; 
return View(vm); 

這裏是查看代碼：

<div class="display-label"> 
    Order Status</div> 
<div class="editor-field"> 
    @Html.DropDownListFor(model => model.StatusID, (SelectList)ViewBag.Statuses) 
    @Html.ValidationMessageFor(model => model.StatusID) 
</div> 

我通過它，甚至在視圖中走它具有正確的SelectedValue但是DDL總是顯示不管選擇的值如何，列表中的第一項。任何人都可以指出我做錯了什麼使DDL默認爲SelectValue？

Answer 1

的SelectList構造（在其中您希望能夠通過所選擇的值id）的最後一個參數，因爲DropDownListFor助手使用您作爲第一個參數傳遞lambda表達式，並使用特定屬性的值將被忽略。

因此，這裏的醜陋的方式做到這一點：

型號：

public class MyModel 
{ 
    public int StatusID { get; set; } 
} 

控制器：

public class HomeController : Controller 
{ 
    public ActionResult Index() 
    { 
     // TODO: obviously this comes from your DB, 
     // but I hate showing code on SO that people are 
     // not able to compile and play with because it has 
     // gazzilion of external dependencies 
     var statuses = new SelectList(
      new[] 
      { 
       new { ID = 1, Name = "status 1" }, 
       new { ID = 2, Name = "status 2" }, 
       new { ID = 3, Name = "status 3" }, 
       new { ID = 4, Name = "status 4" }, 
      }, 
      "ID", 
      "Name" 
     ); 
     ViewBag.Statuses = statuses; 

     var model = new MyModel(); 
     model.StatusID = 3; // preselect the element with ID=3 in the list 
     return View(model); 
    } 
} 

查看：

@model MyModel 
...  
@Html.DropDownListFor(model => model.StatusID, (SelectList)ViewBag.Statuses) 

---

，這裏是正確的方法，用真實的視圖模型：

型號

public class MyModel 
{ 
    public int StatusID { get; set; } 
    public IEnumerable<SelectListItem> Statuses { get; set; } 
} 

控制器：

public class HomeController : Controller 
{ 
    public ActionResult Index() 
    { 
     // TODO: obviously this comes from your DB, 
     // but I hate showing code on SO that people are 
     // not able to compile and play with because it has 
     // gazzilion of external dependencies 
     var statuses = new SelectList(
      new[] 
      { 
       new { ID = 1, Name = "status 1" }, 
       new { ID = 2, Name = "status 2" }, 
       new { ID = 3, Name = "status 3" }, 
       new { ID = 4, Name = "status 4" }, 
      }, 
      "ID", 
      "Name" 
     ); 
     var model = new MyModel(); 
     model.Statuses = statuses; 
     model.StatusID = 3; // preselect the element with ID=3 in the list 
     return View(model); 
    } 
} 

查看：

@model MyModel 
...  
@Html.DropDownListFor(model => model.StatusID, Model.Statuses) 

Answer 2

請從下面的示例代碼。

public class Temp 
    { 
     public int id { get; set; } 
     public string valueString { get; set; } 
    } 

控制器

public ActionResult Index() 
     { 
      // Assuming here that you have written a method which will return the list of Temp objects. 
      List<Temp> temps = GetList(); 

      var tempData = new SelectList(temps, "id", "valueString",3); 
      ViewBag.Statuses = tempData; 

      return View(); 
     } 

查看

@Html.DropDownListFor(model => model.id, (SelectList)ViewBag.Statuses) 
    @Html.ValidationMessageFor(model => model.id) 

Answer 3

爲每個視圖創建一個視圖模型。這樣做，你將只包括屏幕上需要的東西。由於我不知道你在哪裏使用這段代碼，讓我們假設你有一個創建視圖來添加一個新的訂單。

爲您創建視圖中創建一個新的視圖模式：

public class OrderCreateViewModel 
{ 
    // Include other properties if needed, these are just for demo purposes 

    // This is the unique identifier of your order status, 
    // i.e. foreign key in your order table 
    public int OrderStatusId { get; set; } 
    // This is a list of all your order statuses populated from your order status table 
    public IEnumerable<OrderStatus> OrderStatuses { get; set; } 
} 

訂單狀態類：

public class OrderStatus 
{ 
    public int Id { get; set; } 
    public string Name { get; set; } 
} 

在您創建視圖，您將有以下：

@model MyProject.ViewModels.OrderCreateViewModel 

@using (Html.BeginForm()) 
{ 
    <table> 
      <tr> 
       <td><b>Order Status:</b></td> 
       <td> 
        @Html.DropDownListFor(x => x.OrderStatusId, 
         new SelectList(Model.OrderStatuses, "Id", "Name", Model.OrderStatusId), 
         "-- Select --" 
        ) 
        @Html.ValidationMessageFor(x => x.OrderStatusId) 
       </td> 
      </tr> 
    </table> 

    <!-- Add other HTML controls if required and your submit button --> 
} 

你創建操作方法：

public ActionResult Create() 
{ 
    OrderCreateViewModel viewModel = new OrderCreateViewModel 
    { 
      // Here you do database call to populate your dropdown 
      OrderStatuses = orderStatusService.GetAllOrderStatuses() 
    }; 

    return View(viewModel); 
} 

[HttpPost] 
public ActionResult Create(OrderCreateViewModel viewModel) 
{ 
    // Check that viewModel is not null 

    if (!ModelState.IsValid) 
    { 
      viewModel.OrderStatuses = orderStatusService.GetAllOrderStatuses(); 

      return View(viewModel); 
    } 

    // Mapping 

    // Insert order into database 

    // Return the view where you need to be 
} 

當您單擊提交按鈕並將其重定向回錯誤處理的創建視圖時，這將保持您的選擇。

我希望這會有所幫助。

Answer 4

確保您的返回選擇值是一個字符串，而不是在模型中聲明它時爲int。

實施例：

public class MyModel 
{ 
    public string StatusID { get; set; } 
}