Option Strict On
 
Public Class Form1
 
    Private conn As New OleDb.OleDbConnection( _
      "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=D:\Test\VertragsDB.MDB")
    Private ds As New DataSet
    Private initDone As Boolean
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        ReadCompanyIds()
        initDone = True
    End Sub
 
    Private Sub ReadCompanyIds()
        Dim da As New OleDb.OleDbDataAdapter("Select CompanyID, CompanyName" & _
          "From Company Order By CompanyID", conn)
        Try
            da.Fill(ds, "CompanyIDs")
        Catch ex As System.Exception
            MsgBox("DB read failed: " + ex.ToString())
            Return
        End Try
        Dim src As New BindingSource(ds, "CompanyIDs")
        ListBox1.DataSource = src
        ListBox1.DisplayMember = "CompanyName"
    End Sub
 
    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, _
      ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        If Not initDone Then Return
        ReadContacts(CInt(CType(ListBox1.SelectedItem, DataRowView)( _
          "CompanyID")))
    End Sub
 
    Private Sub ReadContacts(ByVal companyId As Integer)
        Dim da As New OleDb.OleDbDataAdapter("Select LastName+', '+FirstName As" & _
          "ContactName From Contacts Where CompanyID=?", conn)
        da.SelectCommand.Parameters.AddWithValue("CompanyID", companyId)
        Try
            If ds.Tables("Contacts") IsNot Nothing Then ds.Tables( _
              "Contacts").Clear()
            da.Fill(ds, "Contacts")
        Catch ex As System.Exception
            MsgBox("DB read failed: " + ex.ToString())
            Return
        End Try
        Dim src As New BindingSource(ds, "Contacts")
        ListBox2.DataSource = src
        ListBox2.DisplayMember = "ContactName"
    End Sub
 
End Class

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Alle Angaben ohne Gewähr. Keine Haftung für Vorschläge, Tipps oder sonstige Hilfe, falls es schiefgeht, nur Zeit verschwendet oder man sonst nicht zufrieden ist