CS0246 Error with aurduino.cs? 'SerialPort' could not be found

[ehansen2580]

I'm trying to build the blinkLED example with Visual Studio 2017. Looks to me like the project is all setup correctly and I've added the aurduino.cs file as per the instructions. But I get the following error with I build:
Severity Code Description Project File Line Suppression State
Error CS0246 The type or namespace name 'SerialPort' could not be found (are you missing a using directive or an assembly reference?) blinkBoard C:\Users\erikj\source\repos\blinkBoard\blinkBoard\arduino.cs 407 Active

I didn't modify the aurduino.cs file at all and just added it to my project. What am I missing?