How Does Nullsafety in C# Fit Into the Type System?

[moneer-muntazah]

Hi

I come from Dart background I want to have a deeper understanding of how null safety works in C#. In Dart, it is easy for me to understand null safety in terms of type hierarchy for example String and Null are subtypes of String? and the ! operator performs a cast operation. How is it different in C# was there a change to the .NET data types hierarchy? When I use the unary ! I'm I casting types? Is the ? operator just an annotation? Or does it state a different type altogether?

Thank you all. I went through the docs, but couldn't get the right answers.

[En3Tho]

It's just a compile-time annotation to help developers be wary of potential nulls. Under the hood string or string? is the same type. "!" is just a "a know better than you, compiler" indicator. There are no casts involved.

It's a different story with Nullable type - it's designed to enable "nullness" for ValueTypes. It's essentially a wrapper - a different type - which holds some value inside it and indicates whether that value was initialized properly or not (kinda "null"). It has a special runtime treatment in some cases like boxing.

So basically "referenceType? (e.g. string?)" is just an compile time annotation and "valueType? (e.g. int?)" is a container type wrapping original value.

[TahirAhmadov]

Also, the ! operator can also be used with int? - the compiler will warn if you do int? x = null; int y = x.Value;, and you can suppress it with int y = x!.Value;