Problèmes de baisse dans .net en utilisant VB et voir le modèle

Question

OK Je suis nouveau à .net, je suis juste prendre une photo dans le noir à travers tout cela. J'ai une table Addresses avec un champ StateID et CountryID. Ils se réfèrent à la table des États et des pays. J'utilise LINQ to SQL et Visual Studio 2010.

Dans mon AddressesController je:

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 

J'ai créé un AddressViewModel et il ressemble à ceci:

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 

Puis dans mon code de vue J'ai:

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

un code supprimé comme inutile

  <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> 

Alors, quand je tente d'aller aux adresses/Créer je reçois cette erreur:

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

Nom du paramètre: articles

Et il pointe cette ligne dans le AddressViewModel:

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

Je sais qu'il tire des résultats, donc j'ai recherché l'élément Parameter et msdn dit que c'est System.Collections.IEnumerable

Alors j'ai changé le AddessViewModel à:

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 

obtenu la même erreur, s'il vous plaît aider

Answer 1

Le problème est la façon dont vous initialisez StateList et CountryList. Ces valeurs sont en cours d'initialisation avant l'appel de l'instruction New. Par conséquent, StateList et CountryList sont vides car _these_States et _these_countries sont vides.

Changer votre code à ceci:

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