Nullable reference types

C# 8+ nullable reference types (NRT) add compile-time warnings for possibly-null strings and objects—closing a gap where reference types could always be null at runtime like in Java. Value types already had int? syntax.

Annotations

#nullable enable
string name = GetName();      // warning if GetName may return null
string? maybe = GetName();    // explicitly nullable
if (maybe is not null) {
    Console.WriteLine(maybe.Length);
}

The compiler tracks flow state after null checks—pattern matching integrates cleanly.

Null-forgiving operator

string sure = GetName()!;  // suppress warning—use sparingly

Prefer guard clauses and validation over sprinkling !.

Important interview questions and answers

1. Q: string vs string?
   A: string? tells the compiler null is allowed; plain string is non-null by contract when NRT is enabled.
2. Q: Do NRTs prevent NullReferenceException?
   A: No—they are warnings/analysis aids; runtime nulls still throw unless you check.

Self-check

1. What suffix marks a nullable reference type?
2. When is the ! operator appropriate?

Tip: Enable #nullable enable in new files—string? documents possible null; plain string is non-null by contract.