The current implementation of the Code128 symbology class used with RadBarcode throws an InvalidSymbolException for characters with codes greater than 127.

As you can see from this table, there are many special characters in Code128 that have larger codes:

https://en.wikipedia.org/wiki/Code_128#Bar_code_widths

It would be useful to have these in applications which need to modify a reader device's behavior with barcode control characters in order to communicate properly.

This was reported in the Code11 and Code39 symbologies.

In the old RadBarcode39 and RadBarcode11 controls the minimum width where the barcode cannot be read any longer is around 180px in the specific scenario. 

In the new RadBarcode and its Code11 and Code39 symbologies, the minimum width is different - around 270px. This reproduces when the SizingMode of the symbology is set to Stretch.

To work this around, you can set the SizingMode of the symbology to Snap. Or alternatively, you can use a bigger Width value for the barcode control, and then apply a ScaleTransform to resize it to the smaller size.

private void RadBarcode_Loaded(object sender, RoutedEventArgs e)
{
    var barcode = (RadBarcode)sender;
    var desiredWidth = 250;
    double relativeWidthDelta = desiredWidth / barcode.ActualWidth;
    barcode.LayoutTransform = new ScaleTransform() { ScaleX = relativeWidthDelta };
}