C# 9.0 quiz medium level

Test description

This test checks the knowledge and coding skills of C# developers who are up-to-date with the features in C# 9 at a medium level. Most of the questions include pieces of code that evaluate understanding of enhancements to the C# 9 language.
Topics: immutability, records, top-level statements, native types, inheritance, pattern matching.

List<int> numbers = null; 
int? i = null; 
numbers ??= new List<int>(); 
numbers.Add(i ??= 17); 
numbers.Add(i ??= 20); 
Console.WriteLine(string.Join(" ", numbers));