Dafür gibt's sogenannte Typecasts:
Delphi-Quellcode:
if MyLayers.Layer[idx] is TBitmapLayer then
TBitmapLayer(MyLayers.Layer[idx]).TuWasMitDemBitmap;
else if MyLayers.Layer[idx] is TTextLayer then
TTextLayer(MyLayers.Layer[idx]).Text := 'Neuer Text';