Dim obj As Object = sender.Textbox     ?????
        Dim txt As String = obj.Text

 
Private Sub Datum_des_Aufnameblattes1Textbox_spezial_LostFocus(ByVal sender As _
  Object, ByVal e  As   System.EventArgs) Handles _
  Datum_des_Aufnameblattes1Textbox_spezial.LostFocus
 
        Dim obj As Object = Datum_des_Aufnameblattes1Textbox_spezial
        Dim txt As String = obj.Text
 
        If Not IsDate(txt) Or txt.Length <> 10 Or txt.Contains(".") Then
            MsgBox("Falsche Eingabe", MsgBoxStyle.Information)
            obj.Focus()
        End If
 
End Sub