จากกระทู้ที่บอกว่าเปลี่ยนค่าใน combobox แล้ว error ก็ยังแก้ไม่ได้เลยคับ(vb.net)

Imports System.Data.SqlClient
Imports System.Data.SqlClient.SqlDataReader

Public Class Form1
    Public conn As New SqlConnection("server=NOR-PC;Integrated Security=True;User Id=sa ;Password=1234;database=test_top;Trusted_Connection=False; ")
    Dim ch_load As Boolean = False
    Public Sub load_order()
        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        conn.Open()
        Dim str As String = "select * from Orders"
        Dim dr As SqlDataReader
        Dim dt As DataTable
        Dim com As SqlCommand
        com = New SqlCommand()
        With com
            .CommandType = CommandType.Text
            .CommandText = str
            .Connection = conn
            dr = .ExecuteReader()
            If dr.HasRows Then
                dt = New DataTable
                dt.Load(dr)
                With cmb_order
                    .ValueMember = "ID_order"
                    .DisplayMember = "ID_order"
                    .DataSource = dt
                End With
            Else
                cmb_order.DataSource = Nothing
            End If
        End With
        conn.Close()

    End Sub
    Public Sub load_deptment()

        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        conn.Open()
        Dim str As String = "select * from Deptment"
        Dim dr As SqlDataReader
        Dim dt As DataTable
        Dim com As SqlCommand
        com = New SqlCommand()
        With com
            .CommandType = CommandType.Text
            .CommandText = str
            .Connection = conn
            dr = .ExecuteReader()
            If dr.HasRows Then
                dt = New DataTable
                dt.Load(dr)
                With cmb_dept
                    .ValueMember = "ID_dept"
                    .DisplayMember = "Name"
                    .DataSource = dt
                End With
            Else
                cmb_order.DataSource = Nothing
            End If
        End With
        conn.Close()

    End Sub
    Public Sub load_employee()
    End Sub
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        load_order()
        load_deptment()
        ch_load = True
    End Sub

    Private Sub cmb_order_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_order.SelectedIndexChanged
        If Not ch_load Then
            Exit Sub
        End If

        Dim str As String = "select Deptment.Name,employee.Name from Orders,employee,Deptment where ID_order=" & cmb_order.SelectedValue.ToString
        str &= " and Orders.ID_emp=employee.ID_emp and Deptment.ID_dept=employee.ID_dept"
        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        conn.Open()
        Dim dr As SqlDataReader
        Dim com As SqlCommand
        com = New SqlCommand()
        With com
            .CommandType = CommandType.Text
            .CommandText = str
            .Connection = conn
            dr = .ExecuteReader()
            dr.Read()
            If dr.HasRows Then
                cmb_dept.Text = dr(0)
                cmb_emp.SelectedValue = dr(1)
            End If
        End With
        conn.Close()

    End Sub

    Private Sub cmb_dept_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_dept.SelectedIndexChanged
        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        conn.Open()
        Dim str As String = "select * from employee where ID_dept=" & cmb_dept.SelectedValue.ToString
        Dim dr1 As SqlDataReader
        Dim dt As DataTable
        Dim com As SqlCommand
        com = New SqlCommand()
        With com
            .CommandType = CommandType.Text
            .CommandText = str
            .Connection = conn
            dr1 = .ExecuteReader()
            If dr1.HasRows Then
                dt = New DataTable
                dt.Load(dr1)
                With cmb_emp
                    .ValueMember = "ID_emp"
                    .DisplayMember = "Name"
                    .DataSource = dt
                End With
            Else
                cmb_order.DataSource = Nothing
            End If
        End With
        dr1.Close()
        conn.Close()
    End Sub
End Class