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Null check

Learn null checking with if (x != null).

Null check

The safest null handling is the if (x != null) check. Inside this block x is treated as non-nullable; this is called type promotion.

Example: String? name = 'Ayşe'; if (name != null) { print(name.length); }. Inside the block name is non-null, so .length is safe.

This pattern is safer than ! or ?. because it checks explicitly and guarantees the type inside the block. In complex conditions Dart may narrow the guarantee; then ! helps.

Limits of promotion

Type promotion works in most cases but has limits. If a variable can be mutated inside a lambda, Dart may drop the guarantee inside the block. Then ! or a local copy is needed.

Example: String? name = 'Ayşe'; if (name != null) { var f = () => print(name); }. Here the lambda could mutate name later, so Dart drops the guarantee.

In practice this is rare. Simple if (x != null) blocks promote safely. For complex cases ! or a local copy can rescue you.

Practice Length with null check

Declare a String? name and assign 'Ayşe'. Check with if (name != null); inside the block add a print(name.length) call.

Starter code

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

  // null check with if and print length
}

Sample solution

main.dart
void main() {
  String? name = 'Ayşe';
  if (name != null) {
    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 is name inside if (name != null)?

  • String (non-nullable) Correct answer
  • null
  • Still String?
  • Throws

Explanation: after the null check the type is non-nullable.

2 Match the null methods

Term Meaning
if (x != null) Type promotion
?. Safe access
! Null assertion
?? Default value

3 Null check with if

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

Explanation: != sets the 'not null' condition.

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