Dart Assert

✅ What is Assert?

Assert statements help you debug code by checking if conditions are true during development. They throw errors when conditions fail, helping you catch bugs early in development.


// Basic assert example
int age = 25;
assert(age >= 0, 'Age cannot be negative');
print('Age is valid: $age');

Assert Features

🔍

Debug Tool

Only works in debug mode

assert(condition);
// Ignored in production

💬

Custom Messages

Add helpful error messages

assert(x > 0, 
  'x must be positive');

⚡

Fast Validation

Quick condition checking

assert(list.isNotEmpty);
assert(name != null);

🛡️

Safety Check

Prevent invalid states

assert(index >= 0 && 
  index < list.length);

🔹 Basic Assert Usage

Simple assert statements for common validations:

void main() {
  int score = 85;
  String name = 'John';
  List numbers = [1, 2, 3];
  
  // Basic assertions
  assert(score >= 0 && score <= 100);
  assert(name.isNotEmpty);
  assert(numbers.length > 0);
  
  print('All validations passed!');
}

Output (Debug Mode):

All validations passed!

🔹 Assert with Messages

Add descriptive messages to make debugging easier:

void calculateArea(double width, double height) {
  assert(width > 0, 'Width must be positive, got: $width');
  assert(height > 0, 'Height must be positive, got: $height');
  
  double area = width * height;
  print('Area: $area');
}

void main() {
  calculateArea(5.0, 3.0);  // Works fine
  // calculateArea(-2.0, 3.0);  // Would throw assertion error
}

Output:

Area: 15.0

🔹 Common Assert Patterns

Typical ways to use assert in your code:

🔸 Function Parameters

void divide(int a, int b) {
  assert(b != 0, 'Cannot divide by zero');
  print('Result: ${a / b}');
}

void setAge(int age) {
  assert(age >= 0 && age <= 150, 'Invalid age: $age');
  print('Age set to: $age');
}

🔸 List Operations

void getItem(List items, int index) {
  assert(items.isNotEmpty, 'List cannot be empty');
  assert(index >= 0 && index < items.length, 
         'Index $index out of bounds');
  
  print('Item: ${items[index]}');
}

🔹 Assert vs Exceptions

Understanding when to use assert vs throwing exceptions:

Use Assert for:

Development debugging: Catch logic errors early
Internal validation: Check your own code assumptions
Performance: No cost in production builds

Use Exceptions for:

User input validation: Handle invalid user data
Runtime errors: Network failures, file not found
Production code: Errors that can occur in release

void processUserInput(String input) {
  // Use exception for user input
  if (input.isEmpty) {
    throw ArgumentError('Input cannot be empty');
  }
  
  List parts = input.split(',');
  
  // Use assert for internal logic
  assert(parts.isNotEmpty, 'Split should never return empty list');
  
  print('Processing: $parts');
}

🔹 Enabling/Disabling Asserts

Control when asserts are active:

Assert Modes:

Debug Mode: Asserts are enabled (default in development)
Release Mode: Asserts are ignored (production builds)
Custom: Use --enable-asserts flag to control

void main() {
  // This will only run in debug mode
  assert(() {
    print('Debug mode is active!');
    return true;
  }());
  
  print('Program continues...');
}