I am developing an app on Visual Studio with mono for Android C#.
How can I send data by USB port in Android C#?
The type serialPort1.Write("1");
Long story short - you can not do it the way you intended. USB is not a serial port that you can just write to and data comes out on the other side.
Serial ports can be emulated over USB, but Android does not support that. Android typically can act as device that offers different profiles: disk, camera and proprietary debug (if enabled on device).
Since Android 3.1 you also have two special modes: host mode where Android device acts as a host to usb devices (much like PC does) and special proprietary accessory mode. Read more about it here: http://developer.android.com/guide/topics/usb/index.html