Dim res As IEnumerable(Of class_kunden_daten) = (From kunde In ds.Tables( _
  "Kunden").AsEnumerable() Join fs In ds.Tables("Familienstand").AsEnumerable() _
  On kunde.Field(Of Nullable(Of Integer))("fs_id") Equals fs.Field(Of Integer)( _
  "fs_id") Where kunde.Field(Of Integer)("Kunde_ID") = Kunde_id
Select New class_kunden_daten With {
 .geburtsdatum = kunde.Field(Of Date)("Geburtsdatum"),
 .kunde_nachname = kunde.Field(Of String)("Nachname"),
 .kunde_vorname = kunde.Field(Of String)("Vorname"),
 .Familienstand = fs.Field(Of String)("Familienstand")}).ToList