InvalidOperationException未处理3

Question

我在VB.Net的LogIn表单中。但是当我尝试登录时出错。 错误显示

  

InvalidOperationException未处理

我正在使用Microsoft Access数据库。

当我运行程序时，它没有说出哪条线路有错误 请帮助我...我非常感谢你，并提前感谢。

以下是我的代码：

Imports System.Data.OleDb
Public Class login
Sub koneksi()
    conn = New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\bin\Debug\db.accdb;Persist Security Info=True")
    conn.Open()
End Sub

Private Sub login_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    koneksi()
End Sub

Private Sub btnlogin_Click(sender As Object, e As EventArgs) Handles btnlogin.Click
    If txtstaffid.Text = "" Or txtpassword.Text = "" Then
        MsgBox("Please insert your staff ID and password")
        txtstaffid.Focus()
        Exit Sub
    Else
        cmd = New OleDbCommand("select * from user where staffid='" & Rep(txtstaffid.Text) & "' and pass='" & Rep(txtpassword.Text) & "'", conn)
        rd = cmd.ExecuteReader          ' Error occurs here !!!
        rd.Read()
        If rd.HasRows Then
            MsgBox("Successful Login!")
            Me.Visible = False
            menuUtama.Show()
            menuUtama.Label1.Text = rd("role")
            Clear()
        Else
            MsgBox("Wrong staff ID or password")
            Clear()
        End If
    End If
End Sub

Sub clear()
    txtstaffid.Text = ""
    txtpassword.Text = ""
End Sub

这是我的module1.vb：

Imports System.Data.OleDb

Module Module1

Public conn As OleDbConnection
Public cmd As OleDbCommand
Public rd As OleDbDataReader

Public Function Rep(ByVal kata As String) As String
    Rep = Replace(kata, "'", "''")
End Function
End Module

错误显示在

行

rd = cmd.ExecuteReader