Use BaseBarcode.Render method:
private void pd_PrintLabel(object sender, PrintPageEventArgs ev)
{
ev.Graphics.PageUnit = GraphicsUnit.Pixel;
BaseBarcode b = BarcodeFactory.GetBarcode(Symbology.Code128);
b.Number = "1999999999999";
b.ChecksumAdd = true;
b.Height = CentimetersToPixels(
2.5,
ev.Graphics.DpiY); // 2.5 cm
b.NarrowBarWidth = CentimetersToPixels(
0.05,
ev.Graphics.DpiX); // 0.05 cm = 0.5 mm
b.Render(
ev.Graphics,
CentimetersToPixels(1.0, ev.Graphics.DpiX),
CentimetersToPixels(1.0, ev.Graphics.DpiY));
}