SerialPort not found

SerialPort not found? Anyone see why? I have added System.IO as a reference but no luck! Using G120

using System;
using Microsoft.SPOT;
using GHI.Premium.IO;
using mfnet.smallbiz.common;
using Microsoft.SPOT.IO;
using System.Threading;
using System.Collections;
using GHI.Premium.Hardware;
using Microsoft.SPOT.Hardware;
using GHI.Hardware.G120;
using System.Text;
using System.IO;

 SerialPort xbee = new SerialPort("COM3", 115200, Parity.None, 8, StopBits.One);

Go to Google and type in the following:

serialport micro framework 4.2

The name of the assembly is


Thank You Architect and Mike…was was this not obvious to me?

Documentation shows the assembly name.
Unfortunately this is one of the very few classes for which assembly information is not correct in the documentation:

it should be: