Cannot drop database "ManfER" because it is currently in use.

make sure your using using(...) for connection and command object because without using using if your opening closing connection from connection object multiple times then also you will get this execption.

Updated:

Adding one more point: you can use DataAdapter this will take care of opening and closing connections. Pros is no need to warry about connection opening closing. Cons is it is slower compared to DataReader.  Also agree with @Amit Gupta solution.