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The ! operator

Learn null assertion with !.

The ! operator

The ! operator performs a null assertion: you say 'I guarantee this value is not null'. name!.length treats name as non-nullable.

If name is actually null, it throws at runtime. So ! is used carefully: only when you are sure the value is non-null.

? and ! are opposites: ? makes a type nullable, ! uses a nullable as non-nullable. In practice null checks are preferred over !, but the bang is sometimes a handy shortcut.

! vs ?.

Both ! and ?. access a member on a nullable value, but they behave differently. ! asserts non-null; if wrong, it throws. ?. returns null when the value is null; no error.

Example: name!.length crashes if name is null; name?.length returns null instead. Which one to pick depends on your task.

Rule of thumb: ! when sure, ?. when not. ! is faster but risky; ?. is safe but yields a nullable result.

Practice Length with !

Declare a String? name and assign 'Ayşe'. Print its length with print(name!.length).

Starter code

main.dart
void main() {
  // declare a nullable and assign 'Ayşe'

  // print length with !
}

Sample solution

main.dart
void main() {
  String? name = 'Ayşe';
  print(name!.length);
}

Expected output

4

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does name!.length do when name is null?

  • Returns null
  • Returns 0
  • Throws an error Correct answer
  • Nothing

Explanation: ! throws on a null value.

2 Assertion with !

Question 
void main() {
  String? name = 'Ayşe';
  print(name____.length);
}
Answer 
void main() {
  String? name = 'Ayşe';
  print(name!.length);
}

Explanation: ! treats the nullable as non-nullable.

3 Match the operators

Term Meaning
! Null assertion
?. Safe access
?? Default value
? Nullable type

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Pattern matching
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. for-in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

  1. Set
  2. Set operations
  3. Map
  4. Map operations
  5. Map iteration
  6. Summary - Sets and Maps

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

  1. class
  2. Constructor
  3. Instance variables
  4. Methods
  5. this
  6. Summary - Class basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together