Ошибка: error C2871: 'System' : a namespace with this name does not exist

Ошибка: error C2871: 'System' : a namespace with this name does not exist

Чтобы исправить выбираем свойства проекта:

Project ->  properties -> General -> Common Language Runtime Support

Common Language Runtime Support устанавливаем = Common Language Runtime Support (/clr)