I understand and thank you PaulS, but if i comment out like
The Open() still resets the Arduino. You need separate buttons to Open() and Close() the serial port.
The Arduino can echo what is sent. C# can have callback methods that get called when serial data is received. You can, in that callback, write the data to the Console, to see what the Arduino received.
There does not, though, appear to be anything wrong with the sending code.