I have chondromalacia patellae(bad knees) can i do dancing?

Yes, but do some PT: You would benefit from physical therapy to optimize strength/endurance/flexibility/coordination in your lower extremities and core; taping or bracing may help for more strenuous activity; see a sports med/knee specialist for specific recs re how to stay active and keep your knees pain free; make your rehab exercises part of your regular exercise routine and you should be able to go dancing.

Might be a prob: Treatment consists of activity modification, ice, anti-inflammatory meds and therapy aimed at flexibility and strengthening, especially of your core. Dancing may cause symptoms to worsen. Try above and slowly introduce dancing. If symptoms worsen, then refrain a bit longer.