Misdirection. Sometimes it's not actually a .NET problem:

If you're using Mono's System.IO.Ports.SerialPort on Linux, and you're getting the System.IO.IOException "Invalid argument", it might not actually be the class library implementation. Check your serial adapter (assuming you're using a USB-to-serial adapter...) I'm using an adapter with the MosChip 7720 chipset:

usbcore: registered new interface driver usbserial
USB Serial support registered for generic
usbcore: registered new interface driver usbserial_generic
usbserial: USB Serial Driver core
USB Serial support registered for Moschip 2 port adapter
mos7720: 1.0.0.4F:Moschip USB Serial Driver
moschip7720 2-1:1.0: Moschip 2 port adapter converter
detected
usb 2-1: Moschip 2 port adapter converter now attached to
ttyUSB0
usb 2-1: Moschip 2 port adapter converter now attached to
ttyUSB1
usbcore: registered new interface driver moschip7720

When opening the serial port, System.IO.Ports.SerialPort indirectly calls SetSignal, which calls the TIOCMGET ioctl, which, as it happens, ISN'T IMPLEMENTED in the mos7720 driver! Harumph.

There's a patch, signed off by both Alan Cox and Greg Kroah-Hartman, but it doesn't seem to be in the upstream, yet.

Patch: Fix the tiocmget/mset handling on the mos7720 USB serial port.