Error on returning list of string vb.net desktop application

i'm trying to create a ntier desktop application using vb.net 2008.I'm trying this code

Presentation Layer

Imports System
Imports System.ComponentModel
Imports System.Data
Imports System.Data.OleDb
Imports System.Drawing
Imports System.Text
Imports System.Windows.Forms
Imports Pos.Pos.BLL
Imports Pos.Pos.DAL
Imports System.Xml
Imports System.Collections.Generic

Public Class frmLogin
Private user As New User()
Private CnnStr As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data " + "Source= " + Application.StartupPath + "\pos.accdb;Jet OLEDB:Database Password=pos"

Private cnn As New OleDbConnection()
Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click
    'Try

    'menu = user.getMenu()
    Dim menu As New List(Of String)
    If cnn.State = ConnectionState.Open Then
        cnn.Close()
    End If
    Try
        Dim jabatan As String = user.login(txtUser.Text, txtPassword.Text)
        MsgBox(jabatan)
        menu = user.getMenu(jabatan)
        'Dim strCommand As String = "select Menu from tblJabatan where JabatanName='" + jabatan + "'"
        'cnn.ConnectionString = CnnStr
        'cnn.Open()
        'Dim Comm As New OleDbCommand(strCommand, cnn)
        'Dim dr As OleDbDataReader = Comm.ExecuteReader
        'Do While dr.Read
        'menu.Add(dr(0).ToString)
        'Loop
        If Not menu Is Nothing Then
            For Each Str As String In menu
                MsgBox(Str)
            Next
        Else
            MsgBox("Tidak ditemukan data")
        End If
    Catch ex As Exception
        Throw ex
    End Try
End Sub
End Class

Business Layer

Public Class User
    Private User As New daUser()
    Private m_UserID As Integer
    Private m_User As String
    Private m_Pwd As String
    Private m_Jabatan As String

    Public Sub New()
        MyBase.New()
    End Sub
    '''<summary>
    '''ID User
    '''</summary>
    Public Property UserID() As Integer
        Get
            Return m_UserID
        End Get
        Set(ByVal value As Integer)
            m_UserID = value
        End Set
    End Property

    '''<summary>
    '''Username
    '''</summary>
    Public Property Username() As String
        Get
            Return m_User
        End Get
        Set(ByVal value As String)
            m_UserID = value
        End Set
    End Property

    '''<summary>
    '''Password
    '''</summary>
    Public Property Password() As String
        Get
            Return m_Pwd
        End Get
        Set(ByVal value As String)
            m_Pwd = value
        End Set
    End Property
    '''<summary>
    '''Jabatan
    '''</summary>
    Public Property Jabatan() As String
        Get
            Return m_Jabatan
        End Get
        Set(ByVal value As String)
            m_Jabatan = value
        End Set
    End Property


    Public Sub add()

        User.add(Me)
    End Sub

    Public Function login(ByVal Username As String, ByVal Password As String) As String
        Return User.Login(Username, Password)
    End Function

    Public Function getMenu(ByVal Jabatan As String) As List(Of String)
        Ret开发者_开发百科urn User.getMenu(Jabatan)
    End Function

End Class

Data Layer

Public Class daUser
Public Sub New()
        cnn.ConnectionString = CnnStr
        cnn.Open()
    End Sub
  Public Function Login(ByVal username As String, ByVal password As String) As String
        'Dim role As String
        If cnn.State = ConnectionState.Open Then
            cnn.Close()
        End If
        Try
            Dim strCom As String = "select Jabatan from tblUser where Username='" + username + "' and Password='" + password + "'"
            cnn.ConnectionString = CnnStr
            cnn.Open()
            Dim Comm As New OleDbCommand(strCom, cnn)
            Dim dr As OleDbDataReader = Comm.ExecuteReader
            While dr.Read
                role = dr(0).ToString
            End While
            cnn.Close()

        Catch ex As Exception
            Return ex.ToString
        Finally
        End Try
        Return role
    End Function

    Public Function getMenu(ByVal jabatan As String) As List(Of String)
        If cnn.State = ConnectionState.Open Then
            cnn.Close()
        End If
        Try
            Dim strCommand As String = "select Menu from tblJabatan where JabatanName='" + jabatan + "'"
            cnn.ConnectionString = CnnStr
            cnn.Open()
            Dim Comm As New OleDbCommand(strCommand, cnn)
            Dim dr As OleDbDataReader = Comm.ExecuteReader
            While dr.Read
                menu.Add(dr("Menu").ToString)
            End While
            cnn.Close()

        Catch er As OleDb.OleDbException


        Catch ex As Exception

        Finally
        End Try
        Return menu
    End Function
    End Class

When i run it,it gives me this warning

System.NullReferenceException: Object reference not set to an instance of an object at Pos.frmLogin.btnLogin_Click(Object sender,EventArgs e) in C:\Documents and Settings\Administrator\My Documents\Visual Studio 2008\Projects \POS\Pos\Form\frmLogin.vb:line 39

Please point me what mistake did i made and how to solve it

The code for the the error is

For Each Str As String In menu
MsgBox(Str)
Next

By the way what i want to achieve is how to get the data as a List(Of String),because if i put the getMenu function into the presentation layer directly,it gives me the data as i expected..Also if i return as a string instead of List(Of String),it gives me the single data. Thanks...

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This is due to the fact that you haven't instantiated menu. Change the declaration for menu to

Dim menu As New List(Of String)

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Someone should probably tell you that you have quite a few things wrong with this. Firstly and most importantly, building Dynamic SQL like that will make your application vulnerable to SQL injection. The other thing is that exception details should not be returned to the user. By doing this you have basically given the caller an "oracle" to use in conjunction with the SQL vulnerability. With just these two problems, anyone with basic SQL knowledge can do whatever they want to your data.

There are a few other items (empty finally, empty catches, not using "Using", object naming collisions, etc), but the security items are much more important.