Closed Type Hierarchies with records … NOT!
TL;DR: You cannot create closed type hierarchies with record types because of the generated protected copy constructor.
Recap: One Year Using C# 9 with .NET Framework
You can use C# 9 language features with .NET Framework or netstandard. After more than a year, I can recommend it.
Be more productive with different keystrokes: Explicit interface implementation
Implement interfaces explicitly to convey intent, get more help from the compiler and maybe type less.
Be more productive with more keystrokes: seal all the things
Declare your classes (and records!) sealed by default.
Be more productive with fewer keystrokes: Access modifiers
Only declare access modifiers if you do not want the default. This makes the code much more readable and succinct.
Creating POCOs from XSD with a Source Generator
How to write a source generator for generating POCOs from XSD files.
Domain Primitives III: easily creating domain-primitive-based data structures from raw data
Only a couple dozen lines of helper code enable easy construction of validated data structures out of raw data as well as proper error tracking.
Returning Auto-Unregistering Objects From An Instance-Counting Factory
Automatically unregister a disposed instance at the factory by using a wrapper around the instance that executes a factory callback on being disposed.
Domain Primitives II: forcing developers to deal with error cases
Using a static method with a Result return type for creating domain primitives forces developers to deal with error cases.
Gotcha with C# 9 Record inheritance and ToString()
On record types, ToString() and other compiler-synthesized methods cannot be overridden up the inheritance hierarchy in parent records, but must be overridden in the concrete record.