难以将值从Access Db加载到VB表单

Question

我在将访问数据库中的值读入visual basic时遇到了一些困难。我正在使用Access和MS Visual Studio的最新版本。我总是遇到错误'NullReferenceException'并且无法加载记录:( plz help！

这是我的代码

Option Explicit On
Option Strict On

Imports System.Data.OleDb

Public Class frmMain

Private Sub btnDisplayAll_Click(sender As Object, e As EventArgs) Handles btnDisplayAll.Click
    Dim oController As Controller = New Controller
    Dim lsData = oController.displayAll()
    Dim sCustomerDetails As String
    For Each product In lsData
        sCustomerDetails = CStr(product("customer_id"))
        sCustomerDetails = sCustomerDetails & " | " & CStr(product("FirstName"))
        sCustomerDetails = sCustomerDetails & " | " & CStr(product("LirstName"))
        sCustomerDetails = sCustomerDetails & " | " & CStr(product("Address"))
        sCustomerDetails = sCustomerDetails & " | " & CStr(product("PhoneNumber"))
        sCustomerDetails = sCustomerDetails & " | " & CStr(product("eMail"))
        sCustomerDetails = sCustomerDetails & " | " & CStr(product("DateOfBirth"))
        Debug.Print(sCustomerDetails)
    Next
End Sub

Private Sub sProductDetails(p1 As Object)
    Throw New NotImplementedException
End Sub

End Class

Public Class Controller

Public Const CONNECTION_STRING As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=Assignment1DB.accdb"

Public Function displayAll() As List(Of Hashtable)

    Dim lsData As New List(Of Hashtable)
    Dim oConnection As OleDbConnection = New OleDbConnection(CONNECTION_STRING)

    Try
        Debug.Print("Connection string:  " & oConnection.ConnectionString)

        oConnection.Open()
        Dim oCommand As OleDbCommand = New OleDbCommand
        oCommand.Connection = oConnection

        'TODO
        oCommand.CommandText = "SELECT * FROM customer ORDER BY customer_id;"
        oCommand.Prepare()
        Dim oDataReader = oCommand.ExecuteReader()

        Dim htCustomerData As Hashtable
        Do While oDataReader.Read() = True
            htCustomerData("CustomerID") = CStr(oDataReader("customer_id"))
            htCustomerData("FirstName") = CStr(oDataReader("firstname"))
            htCustomerData("LastName") = CStr(oDataReader("lastname"))
            htCustomerData("Address") = CStr(oDataReader("address"))
            htCustomerData("PhoneNumber") = CStr(oDataReader("phone_number"))
            htCustomerData("eMail") = CStr(oDataReader("email"))
            htCustomerData("DateOfBirth") = CStr(oDataReader("dob"))
            lsData.Add(htCustomerData)
        Loop

        Debug.Print("The Records were found.")

    Catch ex As Exception
        Debug.Print("Error: " & ex.Message)
        MsgBox("The records couldn't be found.")
    Finally
        oConnection.Close()
    End Try

    Return lsData

End Function

结束班