WebService Asp.Net Json not serialize a List(Of Class) - json

Question

after a lot of time searching here and in the web, i'm posting my problem and my code for yours teach me how i do this works.
The problem is that the Json not serialize listUsuarios, when i use JavaScriptSerializer the return is "{}" and when i use JsonConvert.SerializeObject the return is "False".
Sorry for my bad english.
Here is the code...
USUARIOS.VB
Imports Microsoft.VisualBasic
Public Class Usuarios
Private _Codigo As Long
Private _Nome As String
Private _Telefone As String
Public Property Codigo As Integer
Get
Return _Codigo
End Get
Set(ByVal value As Integer)
_Codigo = value
End Set
End Property
Public Property Nome() As String
Get
Return _Nome
End Get
Set(ByVal value As String)
_Nome = value
End Set
End Property
Public Property Telefone() As String
Get
Return _Telefone
End Get
Set(ByVal value As String)
_Telefone = value
End Set
End Property
End Class
Public Class ListUsuarios
Private listUsuarios As List(Of Usuarios)
Public Sub New()
listUsuarios = New List(Of Usuarios)
End Sub
Public Sub AddItem(ByVal usuario As Usuarios)
listUsuarios.Add(usuario)
End Sub
Protected Overrides Sub Finalize()
MyBase.Finalize()
End Sub
Public Function Item(ByVal Index As Integer) As Usuarios
Return CType(listUsuarios.Item(Index), Usuarios)
End Function
Public Function Count() As Integer
Return listUsuarios.Count
End Function
End Class
ANDROID.VB
Imports Newtonsoft.Json
Imports System.Configuration
Imports System.Data
Imports System.Data.OleDb
Imports System.Web
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.Xml
' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
<System.Web.Script.Services.ScriptService()> _
<WebService(Namespace:="http://tempuri.org/")> _
<WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<Global.Microsoft.VisualBasic.CompilerServices.DesignerGenerated()> _
Public Class Android
Inherits System.Web.Services.WebService
Dim Dt As DataTable
Dim Da As New OleDbDataAdapter
Dim Cmd As New OleDbCommand
Dim cn As New OleDb.OleDbConnection
<WebMethod()> _
Public Function Android(query As String) As String
Dim listUsuario As ListUsuarios = carregaDadosAccess(query)
Return retornaDadosJSON(listUsuario)
End Function
Private Function getConexaoDB() As OleDbConnection
Try
cn.ConnectionString = ConfigurationManager.ConnectionStrings("MyLocalAccess").ConnectionString
cn.Open()
Return cn
Catch ex As Exception
Throw ex
End Try
End Function
Private Sub closeConexaoDB(ByVal cn As OleDbConnection)
Try
If cn.State = ConnectionState.Open Then
cn.Close()
End If
Catch ex As Exception
Throw ex
End Try
End Sub
Private Function carregaDadosAccess(selectCommmand As String) As ListUsuarios
Dim usuario As Usuarios = Nothing
Dim listUsuario As New ListUsuarios
Dim i As Long = 0
Try
cn = getConexaoDB()
Dim da As New OleDb.OleDbDataAdapter(selectCommmand, cn)
Dim dt As New DataTable
Dim arrImage() As Byte = Nothing
Dim myMS As New IO.MemoryStream
da.Fill(dt)
If dt.Rows.Count > 0 Then
Do
usuario = New Usuarios
usuario.Codigo = dt.Rows(i).Item("key-codigo")
usuario.Nome = dt.Rows(i).Item("dad-descri")
usuario.Telefone = dt.Rows(i).Item("dad-telefo")
listUsuario.AddItem(usuario)
i = i + 1
If i = dt.Rows.Count Then Exit Do
Loop
Else
listUsuario = Nothing
End If
Catch ex As Exception
Throw ex
Finally
closeConexaoDB(cn)
End Try
Return listUsuario
End Function
Private Function retornaDadosJSON(listUsuario As ListUsuarios) As String
'Json serializer do próprio .net
'Dim JsonSerializer As New JavaScriptSerializer
'Return JsonSerializer.Serialize(listUsuario)
'Json serializer James Newton-King
Dim JsonSerializer As String = ""
Return JsonSerializer = JsonConvert.SerializeObject(listUsuario)
End Function
End Class

Answer 1

The first problem is you are returning a value that compares an empty string with the serialized json string and since the json serialized string is not empty. This is where the result of "False" is coming from.
You can fix that problem by changing the method to:
Private Function retornaDadosJSON(listUsuario As ListUsuarios) As String
Return JsonConvert.SerializeObject(listUsuario)
End Function
You can prevent this type of issue in the future by adding the following directives to the top of each file:
Option Explicit On
Option Strict On
or by setting the corresponding values in the project's Compile property page (you can also set these to default values for new projects in Visual Studio's Tools > Options > Projects and Solutions > VB Default).
Option Strict On would have prevented that code from compiling. It would also show you that there is a data type difference between _Codigo and Codigo (one is Long the other is Integer).
The second problem is that ListUsuarios isn't defined well for what you are trying to do with it. It would be much better to inherit from List(Of Usuarios) than your current design.
Changing to the following class will both reduce the code and ensure the data is serialized correctly:
Public Class ListUsuarios
Inherits List(Of Usuarios)
Public Sub AddItem(ByVal usuario As Usuarios)
Me.Add(usuario)
End Sub
End Class