null and nullable reference types

A variable of a reference type can hold null, meaning "no object here". Calling a method on it throws NullReferenceException, the most common crash in C#. New projects turn on nullable reference types.

With them, string means "should never be null" and string? means "might be null". The compiler then warns when you use a string? without checking it first. Take those warnings seriously.

The language gives you tools for this: if (name is null), the null-conditional ?. as in order?.Customer, and the null-coalescing ?? as in name ?? "Guest", which supplies a fallback. Console.ReadLine() returns string?, which is why the compiler warns the first time you use its result. Fix the cause rather than silencing the warning with !.