hi there,
is there any type of small/mini project which deals with serial communication interface based on object oriented approach?

Because later, I will use QT designer to build it.
Thanks.

Have you seen any of these examples? At some point you just have to bite the dust and learn how to manipulate the serial ports so that your program can communicate with whatever is on the other end of the serial cable. A pure OOP library is not going to help you learn how to set the baud rate, stop bits, data bits, handshake, and parity. The value of those must match whatever is on the other end of the serial cable. After that, it's just a matter of reading from and writing to the port. You have to know how to talk to the hardware on the other end of the cable.

If you are going to use QT why are you trying to use C#? AFAIK QT uses a c++ compiler.