All 7 Dart Null Safety questions
More in Dart Null Safety
What is null safety in Dart, and how does it help prevent a common category of runtime error known as a null reference exception? What does the question mark symbol mean when it appears after a type in a Dart variable declaration, such as String?, and how is a nullable type actually different from a regular non nullable type? What do the null aware operators, specifically the null coalescing operator and the null aware access operator, actually do, and how do they let you write more concise code when working with nullable values? How does Dart's late keyword let you declare a non nullable variable that will genuinely be initialized before it is ever actually first used, even though it cannot be given an initial value immediately at the exact point of its declaration? How does Dart's type promotion, sometimes also called flow analysis, let the compiler automatically treat a nullable variable as non nullable within a specific block of code immediately after you have already explicitly checked that it is not null? How does migrating a large, existing pre null safety Dart or Flutter codebase to properly support null safety typically work, and what tools does Dart provide to help automate a significant portion of that migration process?
Advertisement
Back to results for “Error Handling”
Flutter › Dart Null Safety

What is the null assertion operator, written as an exclamation mark, and why should it generally be used carefully and only when you are genuinely certain a specific nullable value cannot actually be null at that particular point?

Intermediate

Answer

The null assertion operator, written as a single exclamation mark placed immediately after a nullable expression, tells the Dart compiler that you are personally certain that specific value is genuinely not null at that particular point in your code, even though its declared type technically still permits it to be null, and while this can be useful in situations where you truly know more about the actual runtime state than the compiler's own static type system can determine on its own, using it incorrectly, meaning the value actually does turn out to be null at that point, throws a genuine runtime error, which somewhat undermines one of the core safety benefits that null safety was specifically designed to provide in the first place.
String? userName = fetchCachedUserName();
print(userName!.toUpperCase()); // Throws if userName is actually null
Real-world example A developer uses the null assertion operator on a cached value they have already explicitly verified is not null just a few lines earlier, while carefully avoiding using that same risky operator on a value whose null status genuinely cannot be reliably guaranteed at that point.

Common follow-ups: What are safer alternatives to using the null assertion operator in most typical situations?;What kind of error does using the null assertion operator on an actual null value throw?

Error Handling & Crash Reporting (Sentry & Crashlytics);Dart Language Basics & Syntax
All 7 Dart Null Safety questions