在.net中使用VB下载麻烦并查看模型

Question

好的我是.net新手，我只是在黑暗中通过整个事情拍摄。我有一个具有StateID和CountryID字段的地址表。他们提到了国家和国家的表格。我使用LINQ to SQL和Visual Studio 2010中

在我AddressesController我：

Function Create() As ActionResult 
     Dim these_states = GetStates() 
     Dim these_countries = GetCountries() 
     Dim viewModel = New AddressViewModel(these_states, these_countries) 
     Return View(viewModel) 
    End Function 

    ' 
    ' POST: /Addresses/Create 

    <HttpPost()> _ 
    Function Create(ByVal this_address As Address) As ActionResult 
     dataContext.Addresses.InsertOnSubmit(this_address) 
     dataContext.SubmitChanges() 
     Return RedirectToAction("Index") 
    End Function 

我创建了一个AddressViewModel，它看起来像这样：

Public Class AddressViewModel 
Public Address As Address 
Private _these_states As System.Linq.IQueryable(Of State) 
Private _these_countries As System.Linq.IQueryable(Of Country) 

Sub New(ByVal these_states As System.Linq.IQueryable(Of State), ByVal these_countries As System.Linq.IQueryable(Of Country)) 
    _these_states = these_states 
    _these_countries = these_countries 
End Sub 


Dim StateList As New SelectList(_these_states, "StateID", "Label", 1) 
Public Property States As SelectList 
    Get 
     Return StateList 
    End Get 
    Set(ByVal value As SelectList) 

    End Set 
End Property 

Dim CountryList As New SelectList(_these_countries, "CountryID", "Label", 1) 
Public Property Countries As SelectList 
    Get 
     Return CountryList 
    End Get 
    Set(ByVal value As SelectList) 
enter code here 

在我看来代码

然后我有：

<%@ Page Title="" Language="VB" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage(Of TotallyAwesomeCRM.AddressViewModel)" %> 

将一些代码移除为不必要

  <div class="editor-label"> 
       State 
      </div> 
      <div class="editor-field"> 
       <%: Html.DropDownList("StateID", Model.States, "1")%> 
       <%: Html.ValidationMessageFor(Function(model) model.StateID) %> 
      </div> 

      <div class="editor-label"> 
       Country 
      </div> 
      <div class="editor-field"> 
       <%: Html.DropDownList("CountryID", Model.Countries, "1")%> 
       <%: Html.ValidationMessageFor(Function(model) model.CountryID) %> 
      </div> 

所以，当我尝试到地址/创建我得到这个错误：

ArgumentNullException was not handled by user code:Value cannot be null. 

参数名称：项目

并将其指向该线路AddressViewModel：

Dim StateList As New SelectList(_these_states, "StateID", "Label", 1) 

我知道它拉结果，所以我查找了项目参数和msdn说它是System.Collections.IEnumerable

所以我改变了AddessViewModel到：

Public Address As Address 
Private _these_states As System.Collections.IEnumerable 
Private _these_countries As System.Collections.IEnumerable 

Sub New(ByVal these_states As System.Collections.IEnumerable, ByVal these_countries As System.Collections.IEnumerable) 
    _these_states = these_states 
    _these_countries = these_countries 
End Sub 

得到了同样的错误，请帮忙

Answer 1

的问题是要初始化StateList和CountryList的方式。这些值是在New语句被调用之前被初始化的。因此，StateList和CountryList是空的，因为_these_States和_these_countries是空的。

你的代码改成这样：

Sub New(ByVal these_states As System.Linq.IQueryable(Of State), ByVal these_countries As System.Linq.IQueryable(Of Country)) 
    _these_states = these_states 
    _these_countries = these_countries 
End Sub 


Dim StateList As SelectList 
Public Property States As SelectList 
    Get 
     If StateList Is Nothing Then 
      StateList = New SelectList(_these_states, "StateID", "Label", 1) 
     End If 
     Return StateList 
    End Get 
    Set(ByVal value As SelectList) 

    End Set 
End Property 

Dim CountryList As SelectList 
Public Property Countries As SelectList 
    Get 
     If CountryList Is Nothing Then 
      CountryList = New SelectList(_these_countries, "CountryID", "Label", 1) 
     End If 
     Return CountryList 
    End Get 
    Set(ByVal value As SelectList) 
     ' enter code here 
    End Set 
End Property