@ Html.DisplayListFor - 視圖模型外鍵下拉列表

Question

我已創建一個視圖模型：

public class CreateChemicalViewModel 
{ 
    public IEnumerable<Site> Sites { get; set; } 
    public Chemical Chemical { get; set; } 
} 

我有一個控制器，使視圖模型的新實例，並將其推到視圖：

[HttpPost] 
    [ValidateAntiForgeryToken] 
    public ActionResult Create(Chemical chemical) 
    { 
     CreateChemicalViewModel viewModel = new CreateChemicalViewModel(); 
     viewModel.Sites = siteRepository.Sites; 
     viewModel.Chemical = chemical; 

     if (ModelState.IsValid) 
     { 
      chemicalRepository.Create(chemical); 
     } 
     else 
     { 
      // Something wrong with the values 
     } 
     return View(viewModel); 
    } 

    public ViewResult Create() 
    { 
     return View(); 
    } 

我想補充一點，滿足站點和化學之間的關係的下拉列表...但我想不通的語法：

@model WebApplication.WebUI.Models.CreateChemicalViewModel 

@{ 
    ViewBag.Title = "Create"; 
    Layout = "~/Views/_Main.cshtml"; 
} 

<h2>Create</h2> 

@using (Html.BeginForm()) 
{ 
    @Html.AntiForgeryToken() 

    <div class="form-horizontal"> 
     <h4>Chemical</h4> 
     <hr /> 
     @Html.ValidationSummary(true) 

     <div class="form-group"> 
      @Html.LabelFor(model => model.Chemical.SiteID, new { @class = "control-label col-md-2" }) 
      <div class="col-md-10"> 
       @Html.DropDownListFor(model => model.Chemical.SiteID, new SelectList(model => model.Sites, new SelectListItem() { Text = Model.Sites.Title, Value = Model.Sites.SiteID }) 
       //@* This line is not working, model.Sites.Title exists but the properties of the Site do not (Title and SiteID for example) *@ 
       @Html.ValidationMessageFor(model => model.Chemical.SiteID) 
      </div> 
     </div> 
...   

}

<div> 
    @Html.ActionLink("Back to List", "List") 
</div> 

任何想法，我需要做的，如果我想使用密鑰=標題和值= SITEID一個下拉列表，其中列出了「站點」，使所選擇的「網站」成爲該值化學關係（SITEID）

---

堆棧

[NullReferenceException: Object reference not set to an instance of an object.] 
    ASP._Page_Views_Chemical_Create_cshtml.Execute() in c:\Users\James\Documents\Visual Studio 2013\Projects\Application\WebApplication.WebUI\Views\Chemical\Create.cshtml:24 
    System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +272 
    System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +121 
    System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +131 
    System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +695 
    System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +382 
    System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +431 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +39 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +116 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +529 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +106 
    System.Web.Mvc.Async.<>c__DisplayClass28.<BeginInvokeAction>b__19() +321 
    System.Web.Mvc.Async.<>c__DisplayClass1e.<BeginInvokeAction>b__1b(IAsyncResult asyncResult) +185 
    System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +42 
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +133 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56 
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +40 
    System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +34 
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +70 
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +139 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40 
    System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +44 
    System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller) +39 
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +62 
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +139 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40 
    System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +39 
    System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +39 
    System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__4(IAsyncResult asyncResult, ProcessRequestState innerState) +39 
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +70 
    System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +139 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59 
    System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40 
    System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +40 
    System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +38 
    System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9688704 
    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155 

Answer 1

試試這個

@Html.DropDownListFor(model => model.Chemical.SiteID, new SelectList(Model.Sites, "SiteID", "Title", Model.Chemical!= null ? Model.Chemical.SiteID : null)) 

編輯： 在你HTTPGET「創建」您需要返回CreateChemicalViewModel，因爲它包含下拉項行動。試試這個

public ViewResult Create() 
{ 
    CreateChemicalViewModel viewModel = new CreateChemicalViewModel(); 
    viewModel.Sites = siteRepository.Sites; 
    return View(viewModel); 
}