CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) code example

Example 1: dot net core 3.1 The type or namespace name 'TestClassAttribute' could not be found (are you missing a using directive or an assembly reference?)

1. Close Visual Studio
2. Delete .vs folder
3. Reopen Visual Studio and it should work.

Example 2: The type or namespace name 'IEnumerator' could not be found (are you missing a using directive or an assembly reference?)

using System.Collections;

Example 3: the type or namespace name 'system' could not be found visual studio 2019

Clean your solution
Do a rebuild
Close visual studio
and re-open

That's it!

Example 4: error CS0246: The type or namespace name 'Slider' could not be found (are you missing a using directive or an assembly reference?)

using UnityEngine.UI;