Month, ZeroableMonth, MonthRange, etc. must be structures instead of classes since they are should be considered ValueTypes

[jochenwezel]

Problem

Dim A as new Month(2020,1)
Dim B as new MonthRange(A, A)
Assert.AreEqual(2020, B.FirstMonth.Year) 'success
A.Year = 2021

Assert.AreEqual(2020, B.FirstMonth.Year) 'failed: B.FirstYear = 2021 instead of 2020
'expected by typical developer: 2020, but is now 2021 since Month is a class and 
'MonthRange was created using a pointer/reference instead of a copy from A

Attention

• existing code might break down if it comes to Null-checks or comparisons with Null-values
• existing compiled code breaks down after replacing dependency: binary compatibility is broken down since class objects are structures, now