DropDownlist ไม่จำค่า มันจะเด้งเป็นค่าแรกตลอด เมื่อคลิก DropDown ตัวที่1 มันจะเด้งเป็นค่าแรกตลอดแล้วจะไม่ส่งค่าไป DropDown ตัวที่ 2

Partial Class test2
    Inherits System.Web.UI.Page
    Dim Conf As System.Configuration.Configuration = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~/")
    Dim Conn As New SqlConnection(Conf.ConnectionStrings.ConnectionStrings("Conn").ToString)

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim sql As String = "SELECT machine_id FROM UBA_machine_property where Machine_id is not null GROUP BY machine_id"
        
        Dim da1 As New SqlDataAdapter(sql, Conn)
        Dim ds1 As New DataSet
        Dim tmp As New ListItem("----", "")
        da1.Fill(ds1)
        ddl1.DataSource = ds1
        ddl1.DataValueField = "machine_id"
        ddl1.DataTextField = "machine_id"
        ddl1.DataBind()
        tmp = New ListItem("----", "")
        ddl1.Items.Insert(0, tmp)
    End Sub

    Protected Sub ddl1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddl1.SelectedIndexChanged

        Dim Strddl1 As String
        Strddl1 = ddl1.SelectedValue.ToString

        Label1.Text = Strddl1

        Dim sql As String = "SELECT property_name_en,machine_id FROM UBA_machine_property where machine_id = '" & Strddl1 & "'"

        Dim da2 As New SqlDataAdapter(sql, conn)
        Dim ds2 As New DataSet
        da2.Fill(ds2)
        ddl2.DataSource = ds2
        ddl2.DataTextField = "property_name_en"
        ddl2.DataValueField = "machine_id"
        ddl2.DataBind()

    End Sub

 End Class