mvc 中没有为此对象定义无参数构造函数

Question

当我在视图中使用 SelectList 时，出现错误“没有为此对象定义无参数构造函数”，我看到了很多解决方案，但找不到适合我的解决方案。这是查看代码：

@using (Html.BeginForm("Product", "Products", FormMethod.Post, new { @class = "form-horizontal" }))
{
   @Html.ValidationSummary()

   <div class="form-group">
       <label class="control-label col-md-3" for="SelectProduct">Select Product</label>
       <div class="col-md-4">
           @Html.DropDownListFor(p => p.SelectProduct, new SelectList(new[] 
           {
             new { Value = "New", Text = "New Entry" },
             new { Value = "Existing", Text = "Existing Entry" },
           }, "Value", "Text"), 
           new { @class = "form-control" })
       </div>
   </div> 
   <div class="form-group">
       <label class="control-label col-md-3" for="productName">Product Name</label>
       <div class="col-md-8">
          @Html.TextBoxFor(u => u.ProductName, new { @class = "form-control", @id = "productName" })
       </div>
   </div>   
}

控制器：

[HttpPost]
public ActionResult Product(ProductDto model)
{
    //code here
}

模型：

public class ProductDto
{
    public ProductDto()
    {
    }
    /// <summary>
    /// Gets or sets whether its New or existing product
    /// </summary>
    public SelectList SelectProduct { get; set; }


    /// <summary>
    /// Gets or sets product name.
    /// </summary>
    public string ProductName { get; set; }

}

堆栈跟踪：

[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, StackCrawlMark& stackMark) +119
      System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) +232
     System.Activator.CreateInstance(Type type, Boolean nonPublic) +83
     System.Activator.CreateInstance(Type type) +11
     System.Web.Mvc.DefaultModelBinder.CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType) +183
    System.Web.Mvc.DefaultModelBinder.BindSimpleModel(ControllerContext controllerContext, ModelBindingContext bindingContext, ValueProviderResult valueProviderResult) +329
     System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +368
     System.Web.Mvc.DefaultModelBinder.GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder) +17
      System.Web.Mvc.DefaultModelBinder.BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor) +384
      System.Web.Mvc.DefaultModelBinder.BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) +88
                  System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +53
         System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +1314
        System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +416
      System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) +317
         System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +117
       System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__1e(AsyncCallback asyncCallback, Object asyncState) +446
       System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130
       System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeAction(ControllerContext controllerContext, String actionName, AsyncCallback callback, Object state) +302
    System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__17(AsyncCallback asyncCallback, Object asyncState) +30
      System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130
  System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) +381
       System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130
      System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +317
        System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +17
       System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__2(AsyncCallback asyncCallback, Object asyncState) +71
   System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +130

谢谢

Answer 1

DropDownListFor意味着在模型中的属性上设置一个值 - 您的模型包含一个SelectProduct类型的属性SelectList- 我猜它应该是类型，因为这是您在视图中构建的属性string的类型。ValueSelectList

public class ProductDto
{
    public ProductDto()
    {
    }
    /// <summary>
    /// Gets or sets whether its New or existing product
    /// </summary>
    public string SelectProduct { get; set; }


    /// <summary>
    /// Gets or sets product name.
    /// </summary>
    public string ProductName { get; set; }

  }

SelectList请注意，更纯粹的方法是在控制器中构建，而ViewModel不是在视图中构建。这样您就可以将构建可能值的逻辑与视图本身分开。