在Post上,下拉列表SelectList.SelectedValue为null

Question

我的模型如下:

public class testCreateModel
{
    public string s1 { get; set; }
    public SelectList DL { get; set; }

    public testCreateModel()
    {
        Dictionary<string, string> items = new Dictionary<string, string>();
        items.Add("1", "Item 1");
        items.Add("2", "Item 2");
        DL = new SelectList(items, "Key", "Value");
    }
}

我的启动行动是:

    public ActionResult testCreate()
    {
        testCreateModel model = new testCreateModel();
        return View(model);
    }

我的剃刀视图(删除了相关部分)是:

@model Tasks.Models.testCreateModel

@using (Html.BeginForm()) {
<fieldset>
    <legend>testCreateModel</legend>

    <div class="editor-label">
        @Html.LabelFor(model => model.s1)
    </div>
    <div class="editor-field">
        @Html.EditorFor(model => model.s1)
    </div>

    <div class="editor-label">
        Select an item:
    </div>
    <div class="editor-field">
        @Html.DropDownList("dropdownlist", (SelectList)Model.DL)
    </div>

    <p>
        <input type="submit" value="Create" />
    </p>
</fieldset>
}

回发行动是:

    public ActionResult testCreate(testCreateModel model, FormCollection collection)
    {
        if (ModelState.IsValid)
        {
            Console.WriteLine("SelectedValue: ",model.DL.SelectedValue);
            Console.WriteLine("FormCollection:", collection["dropdownlist"]);
            // update database here...
        }
        return View(model);
    }

在回发后,model.DL.SelectedValue为null.(但是,可以从FormCollection中获取所选项目,但这不是重点).DL对象仍然正确填充,否则,立即窗口输出如下:

model.DL
{System.Web.Mvc.SelectList}
    base {System.Web.Mvc.MultiSelectList}: {System.Web.Mvc.SelectList}
    SelectedValue: null
model.DL.Items
Count = 2
    [0]: {[1, Item 1]}
    [1]: {[2, Item 2]}
model.DL.SelectedValue
null

Q1:如何使用SelectedValue属性？

现在,如果在Razor视图中我将Html SELECT标记的名称更改为DL(即与模型中的属性名称相同):

@Html.DropDownList("DL", (SelectList)Model.DL)

我得到一个例外:

No parameterless constructor defined for this object. 
Stack Trace: 
[MissingMethodException: No parameterless constructor defined for this object.]
System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck) +0
System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache) +98
System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean skipCheckThis, Boolean fillCache) +241
System.Activator.CreateInstance(Type type, Boolean nonPublic) +69
System.Web.Mvc.DefaultModelBinder.CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) +199
System.Web.Mvc.DefaultModelBinder.BindSimpleModel(ControllerContext controllerContext, ModelBindingContext bindingContext, ValueProviderResult 
...

Q2:为什么？

谢谢.

Answer 1

MVC将仅返回POST中所选选项的值,因此您需要一个属性来包含返回的单个值.

作为一个好建议,尝试通过ViewBag设置SelectLists,这有助于保持ViewModel不受需要填充表单的数据的影响.

所以你的例子可以像这样解决:

public class testCreateModel
{
    public string s1 { get; set; }
    public int SelectedValue { get; set; }
}

并在您的视图中执行此操作:

@Html.DropDownList("SelectedValue", (SelectList)ViewBag.DL)

在您的GET操作中填充ViewBag.DL之前.

至于你的Q2,默认的ModelBinder要求所有要绑定的类型都有一个默认的构造函数(以便ModelBinder可以创建它们)

Answer 2

已经选择了答案,但请看我是如何做到的.下面是填写下拉列表时我通常如何做的代码.这是非常简单的,我建议你用它来建立你的下拉.

在我的视图顶部,我指定了我的视图模型:

@model MyProject.ViewModels.MyViewModel

在我看来,我有一个下拉列表,显示用户可以选择的所有银行:

<table>
     <tr>
          <td><b>Bank:</b></td>
          <td>
               @Html.DropDownListFor(
                    x => x.BankId,
                    new SelectList(Model.Banks, "Id", "Name", Model.BankId),
                    "-- Select --"
               )
               @Html.ValidationMessageFor(x => x.BankId)
          </td>
     </tr>
</table>

我总是有视图的视图模型,我从不直接将域对象传递给视图.在这种情况下,我的视图模型将包含将从数据库填充的银行列表:

public class MyViewModel
{
     // Other properties

     public int BankId { get; set; }
     public IEnumerable<Bank> Banks { get; set; }
}

我的银行域名模型:

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

然后在我的action方法中,我创建了一个视图模型的实例,并从数据库中填充了银行列表.完成后,我将视图模型返回到视图:

public ActionResult MyActionMethod()
{
     MyViewModel viewModel = new ViewModel
     {
          // Database call to get all the banks
          // GetAll returns a list of Bank objects
          Banks = bankService.GetAll()
     };

     return View(viewModel);
}

[HttpPost]
public ActionResult MyActionMethod(MyViewModel viewModel)
{
    // If you have selected an item then BankId would have a value in it
}

我希望这有帮助.