Visual Studio 2012 csc.exe Error. Executable not valid for OS

Question

I am getting the following error when trying to compile and test any type of project in Visual Studio 2012 Professional.

Error 1 The specified task executable \

Answer 1

I am on Visual Studio 2017 but this is the first google result that pops up for "csc.exe not found"

When I got this error I had to restart Visual Studio, being sure to select 'Run As Administrator'. Everything worked after that