One problem related to alternator/generator/regulator has been reported for the 2009 Mercedes Benz C300. The most recently reported issues are listed below.
Common and known issue with m benz C300 eis/els/steering column. Car engine just dies and will not start due to electrical component malfunction. An expensive, possibly dangerous problem that has not been recognized by mercedes-benz it appears from the other comments on this forum and others. My 2009 C300 was running fine. I maintain regular service. Stopped to grab lunch. Fortunately, I was not in motion or in a dangerous area or in the middle of night in the middle of nowhere. The car would not re-start, yet the lights, radio, panel all appeared to be working, although "remove key from ignition" message appeared. Thought it could be a battery or alternator issue. Had to have car towed to 2 different places, the last of which was the mbenz dealership over an hour away. This occurred at 80k miles on odometer. A major safety and possibly dangerous situation. I thought the mb brand was about reliability, quality, safety. So disappointed. Manufacturer's defective product.