Thank you for your quick response.
I added below code. FTPReply is ok(Data channel is clear).
However after establishing connection, when i try to get files from server, i get the error "The server hello message uses a protocol that was not recognized".
Is it something about my ftp server, or just misconfiguring my connection?
FTPReply reply = connection.InvokeFTPCommand("PROT C", "200");
string[] files = connection.GetFiles(directory);