Drop down troubles in .net using VB and view model

Question

OK I am new to .net, I am just taking a shot in the dark through this whole thing. I have an Addresses table with a StateID and CountryID field. They refer to the States and Countries table. I am using LINQ to SQL and Visual Studio 2010.

In my AddressesController I have:

    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

I created an AddressViewModel and it looks like this:

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

Then in my view code I have:

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

some code removed as unnecessary

            <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>

So when I try to go to Addresses/Create I get this error:

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

Parameter name: items

And it points to this line in the AddressViewModel:

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

I know it pulls results, so I looked up the item Parameter and msdn says it is System.Collections.IEnumerable

So I changed the AddessViewModel to:

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

Got the same error, please help

Answer 1

The problem is the way you are initializing StateList and CountryList. These are values are being initialized before the New statement is called. Therefore StateList and CountryList is empty because _these_States and _these_countries are empty.

Change your code to this:

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