Ok so basically I have a login form connected to a database and it (Database) contains my usernames and passwords. I also included a security level so that if a certain member logs on, features to them will be disabled. So For example if Katie logged in she can view all records BUT cannot delete them.

However there is a problem, each time I try to log in the following message appears:

"object reference not set to an instance of an object" either that message appears or this one:

"not allowed to change the 'connectionstring' property. the connection's current state is open."

I've been trying to solve this problem for a while now and I can't fix it.

Below is the code for my login form:



Code:
Imports System.Data.OleDb


Public Class frmLogin

    Dim con As New OleDb.OleDbConnection
    Dim dbProvider As String
    Dim dbSource As String
    Dim ds As New DataSet
    Dim da As OleDbDataAdapter
    Dim sql As String
    Dim dt As New DataTable
    Dim DReader As OleDbDataReader


     Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click


 'Dim dr As DataRow
        Dim dr As OleDbDataReader
        'Dim dt As DataTable
        Dim user As String = Trim(txtUsername.Text)
        Dim pword As String = Trim(txtPassword.Text)

        ' If the username or password fields are empty then this displays an error message:
        If user = "" Then
            MessageBox.Show("Please enter the username.", Me.Text, MessageBoxButtons.OK, MessageBoxIcon.Error)
        ElseIf pword = "" Then
            MessageBox.Show("Please enter the password.", Me.Text, MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If

        If user <> "" And pword <> "" Then

        End If

        dbSource = "Data Source =G:\Assignment 2 codes\ValleyMilk.accdb"

        dbProvider = "PROVIDER = Microsoft.Ace.OLEDB.12.0;"

        Try
            con.ConnectionString = dbProvider & dbSource
            con.Open()
            sql = "SELECT * FROM Users WHERE Username= user AND [Password]= pword"
            Dim cmd As OleDbCommand = New OleDbCommand(sql, con)

            cmd.Parameters.AddWithValue("Username", user)
            cmd.Parameters.AddWithValue("Password", pword)

            dr = cmd.ExecuteReader
            ' If dr.Read() = True And user = dr("Username").ToString And pword = dr("Password").ToString Then
     'LogginUserLevel and LoggedinUser are from Module1.vb

            LoggedinUserLevel = DReader("SecurityLevel")
            LoggedinUser = DReader("Username")

            If dr.Read() = True Then
                MsgBox("Welcome back !")
                Me.Hide()
                frmMainMenu.Show()
            Else
                MsgBox("Login Failure")

                dr.Close()
                con.Close()

            End If

        Catch errObj As Exception

            MessageBox.Show(errObj.Message)
        End Try


Any help on this would be great.