Remove NullableAttributes #20
Description
🆙 Improvement proposal
I added nullable attributes (currently only the NotNullWhenAttribute) in #19, because the project runs on the old .NET framework, and these attributes only exist in the System library for .NET Core.
If this project switches to .NET Core, the attribute should be removed.
Currenlty it's marked as obsolete.
Context
The copy pasted NotNullWhenAttribute in the file 'NullableAttributes.cs' is a temporary solution, this is your reminder to not forget to remove it if you ever get around for a .NET Core migration :).