asp.net-mvc - MVC DropDownList SelectedValue无法正确显示

Question

我尝试搜索，但没有找到解决我问题的方法。我在Razor View 上有一个DropDownList，它不会显示我在SelectList中标记为Selected的项目。这是填充列表的 Controller 代码:

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

这是查看代码:

<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>

我遍历了它，即使在 View 中它具有正确的SelectedValue，但是DDL始终显示列表中的第一项，而不管选择的值如何。谁能指出我做错了什么才能使DDL默认为SelectValue？

Answer 1

SelectList构造函数的最后一个参数(您希望能够传递所选的值id)将被忽略，因为DropDownListFor助手使用您作为第一个参数传递的lambda表达式，并使用特定属性的值。

因此，这是执行此操作的丑陋方式:

模型:

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

Controller :

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);
    }
}

View :

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

这是使用真实 View 模型的正确方法:

模型

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

Controller :

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);
    }
}

View :

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