How do you get Gengar in Pokemon Diamond?

by going into old chateau after finishing poke league and finding the room with the scary painting

or you can trade a medicham with a girl in snowpoint city and she will give you a 35 haunter and you can evolve it from there. this is the harder way
Answer: Go to Old Chateau, in the Eterna forest,go to the left side room (be sure to have repels) go to the the door you will see this painting with red eyes.I dont know how to get it though. Or, 2. Go to the spring path, go this cave, you'll find a cave with millions of Haunters.Catch 1, trade it with a friend, trade back, and TA-DA!! You now got one!! Answer: Go to Old Chateau, in the Eterna forest,go to the left side room (be sure to have repels) go to the the door you will see this painting with red eyes.I dont know how to get it though. Or, 2. Go to the spring path, go this cave, you'll find a cave with millions of Haunters.Catch 1, trade it with a friend, trade back, and TA-DA!! You now got one!!