Not able to create instance of Session class in c#.

public Session OpenSftpSession ()
    {
      SessionOptions sessionOptions = new SessionOptions ();
      sessionOptions.Protocol = WinSCP.Protocol.Sftp;
      sessionOptions.HostName = this.hostName_;
      sessionOptions.UserName = this.userName_;
      sessionOptions.Password = this.password_;
      sessionOptions.SshHostKeyFingerprint = this.sshHostKeyFingerprint_;

      Session session = null;
      try
      {
        session = new Session ();
        session.Open (sessionOptions);
      }
      catch (Exception ex)
      {
        return null;
      }

      return session;
    }

session = new Session ();