C# 下拉列表为'；行不通

控制器：

   OnePersonAllInfoViewModel vModel = new OnePersonAllInfoViewModel();
  vModel.PreferredContactType = new PreferredContactType();


ViewBag.PrefContactTypes = new SelectList(dbEntities.PreferredContactTypes
                                  .OrderBy(pct => pct.PreferredContactTypeID),
                                   "PreferredContactTypeID", "PreferredContactType1",
                                   vModel.PreferredContactType.PreferredContactTypeID);

视图：

@LabelFor（model=>model.PreferredContactType.PreferredContactTypex）
@Html.DropDownListFor（model=>model.PreferredContactType.PreferredContactTypeID，
ViewBag.PrefContactTypes作为选择列表）

我在回邮时收到了这个错误。。。没有具有键“PreferredContactType.PreferredContactTypeID”的“IEnumerable”类型的ViewData项

---

有什么想法吗？谢谢

在HttpPost控制器操作中，如果重新显示相同的视图，则必须以与GET操作相同的方式重新填充

ViewBag.PrefContactTypes

属性：

[HttpPost]
public ActionResult Process(OnePersonAllInfoViewModel model)
{
    ViewBag.PrefContactTypes = ...
    return View(model);
}

您似乎还定义了一些以ViewModel为后缀的类。这让读者相信，您在应用程序中使用的是视图模型，在下一行中使用的是

ViewBag

。为什么？为什么不充分利用视图模型及其强大的类型呢

就这样,

public class OnePersonAllInfoViewModel
{
     public int PreferredContactTypeID { get; set; }
     public IEnumerable<PreferredContactType> PrefContactTypes { get; set; }
}

然后是一种观点：

@Html.DropDownListFor(
    model => model.PreferredContactTypeID, 
    Model.PrefContactTypes
)

行动后：

[HttpPost]
public ActionResult Index(OnePersonAllInfoViewModel model)
{
    if (!ModelState.IsValid)
    {
        // the model is invalid => we must redisplay the same view =>
        // ensure that the PrefContactTypes property is populated
        model.PrefContactTypes = dbEntities
            .PreferredContactTypes
            .OrderBy(pct => pct.PreferredContactTypeID)
            .ToList();
        return View(model); 
    }

    // the model is valid => use the model.PreferredContactTypeID to do some
    // processing and redirect
    ...

    // Obviously if you need to stay on the same view then you must ensure that 
    // you have populated the PrefContactTypes property of your view model because
    // the view requires it in order to successfully render the dropdown list.
    // In this case you could simply move the code that populates this property
    // outside of the if statement that tests the validity of the model

    return RedirectToAction("Success"); 
}

---

回发时，是否已使用数据重新绑定viewModel？此错误意味着vModel.PreferredContactType为空（如果在保存更改或验证后执行回邮并返回相同页面，则此错误为正常）。

@Html.DropDownListFor(
    model => model.PreferredContactTypeID, 
    Model.PrefContactTypes
)

[HttpPost]
public ActionResult Index(OnePersonAllInfoViewModel model)
{
    if (!ModelState.IsValid)
    {
        // the model is invalid => we must redisplay the same view =>
        // ensure that the PrefContactTypes property is populated
        model.PrefContactTypes = dbEntities
            .PreferredContactTypes
            .OrderBy(pct => pct.PreferredContactTypeID)
            .ToList();
        return View(model); 
    }

    // the model is valid => use the model.PreferredContactTypeID to do some
    // processing and redirect
    ...

    // Obviously if you need to stay on the same view then you must ensure that 
    // you have populated the PrefContactTypes property of your view model because
    // the view requires it in order to successfully render the dropdown list.
    // In this case you could simply move the code that populates this property
    // outside of the if statement that tests the validity of the model

    return RedirectToAction("Success"); 
}