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procedure TForm2.Button1Click(Sender: TObject);
var cds : TClientDataSet;
    f : TStringField;
begin

  cds := TClientDataSet.Create(self);
  try
    f := TSTringField.Create(self);
    f.FieldName := 'Test';
    cds.Fields.Add(f);

    cds.CreateDataSet;

    cds.Append;
    cds.FieldByName('Test').AsString := 'Test'; {Hier ist die Zugriffsverletzung}

  finally
    cds.Free;
  end;

end;