ATMEL STUDIO: avrdude.exe: ser_open(): can't open device
I set the arguments line to COM3 like this:


-CC:\Arduino\avrdude.conf -patmega2560 -cstk500v2 -P\\.\ COM3 -b115200 -Uflash:w:"$(ProjectDir)Debug\$(TargetName).hex":i 


This seems to have a mistaken shell escape sequence in the port specification.

Typically the specification to avrdude would be of the form -P COM3

For unclear reasons, you are inserting into this the escape sequence \\.\ which would typically be interpreted as \.\ however in, your case it seems not to be being interpreted by a shell (or else to be being escaped again by Atmel Studio before being passed to one).

The result is clear in your error message:

avrdude.exe: ser_open(): can't open device "\\.\": The filename, directory name, or volume label syntax is incorrect.


avrdude has been lead to believe that the name of the serial device you want to open is the first word following -P which is to say \\.\ - obviously not what you want.

The solution should simply be to specify -P COM3 or whatever the correct port is.

It's unclear what lead to the belief that the prefixing sequence you were trying to use would be appropriate elsewhere, but it is clearly inappropriate in this setting.