Dart UnmodifiableSetView

last modified April 4, 2025

In Dart, UnmodifiableSetView is an immutable view of a Set that prevents modifications to the underlying collection. It wraps an existing Set.

UnmodifiableSetView implements the Set interface but throws exceptions for modification operations. It's useful for providing read-only access to sets.

Creating an UnmodifiableSetView

The simplest way to create an UnmodifiableSetView is using the constructor.

import 'dart:collection';

void main() {
  var numbers = {1, 2, 3};
  var unmodifiableView = UnmodifiableSetView(numbers);
  
  print(unmodifiableView);
  
  try {
    unmodifiableView.add(4); // Will throw
  } catch (e) {
    print('Error: $e');
  }
}

We create an UnmodifiableSetView from a regular Set. Attempting to modify it throws an UnsupportedError. The view reflects changes to the original Set.

$ dart main.dart
{1, 2, 3}
Error: Unsupported operation: Cannot modify an unmodifiable set

UnmodifiableSetView from Set.unmodifiable

Dart's Set.unmodifiable factory creates an UnmodifiableSetView internally.

import 'dart:collection';

void main() {
  var colors = {'red', 'green', 'blue'};
  var unmodifiableColors = Set.unmodifiable(colors);
  
  print(unmodifiableColors.runtimeType);
  print(unmodifiableColors);
  
  // Changes to original don't affect the unmodifiable copy
  colors.add('yellow');
  print(unmodifiableColors);
}

Set.unmodifiable creates a snapshot of the original Set. Unlike UnmodifiableSetView, changes to the original Set don't affect the copy.

$ dart main.dart
UnmodifiableSetView<String>
{red, green, blue}
{red, green, blue}

Checking Contents

UnmodifiableSetView supports all read operations of the Set interface.

import 'dart:collection';

void main() {
  var fruits = {'apple', 'banana', 'orange'};
  var view = UnmodifiableSetView(fruits);
  
  print('Contains apple: ${view.contains('apple')}');
  print('Length: ${view.length}');
  print('First element: ${view.first}');
  print('Last element: ${view.last}');
  print('Is empty: ${view.isEmpty}');
}

We demonstrate various read operations available on UnmodifiableSetView. These operations work the same as on regular Sets but are guaranteed to be safe.

$ dart main.dart
Contains apple: true
Length: 3
First element: apple
Last element: orange
Is empty: false

Iterating Over UnmodifiableSetView

UnmodifiableSetView supports all standard Set iteration methods.

import 'dart:collection';

void main() {
  var primes = {2, 3, 5, 7, 11};
  var primeView = UnmodifiableSetView(primes);
  
  print('Using for-in:');
  for (var prime in primeView) {
    print(prime);
  }
  
  print('\nUsing forEach:');
  primeView.forEach(print);
  
  print('\nUsing where:');
  var largePrimes = primeView.where((p) => p > 5);
  print(largePrimes);
}

We demonstrate three ways to iterate through an UnmodifiableSetView. The view provides the same iteration capabilities as a regular Set but is immutable.

$ dart main.dart
Using for-in:
2
3
5
7
11

Using forEach:
2
3
5
7
11

Using where:
(7, 11)

Set Operations

UnmodifiableSetView supports non-modifying Set operations like union and intersection.

import 'dart:collection';

void main() {
  var setA = UnmodifiableSetView({1, 2, 3, 4});
  var setB = UnmodifiableSetView({3, 4, 5, 6});
  
  print('Union: ${setA.union(setB)}');
  print('Intersection: ${setA.intersection(setB)}');
  print('Difference: ${setA.difference(setB)}');
  
  print('\nLookups:');
  print('Contains 3: ${setA.contains(3)}');
  print('Lookup 5: ${setA.lookup(5)}');
}

These operations return new Sets rather than modifying the existing view. The original UnmodifiableSetView remains unchanged after these operations.

$ dart main.dart
Union: {1, 2, 3, 4, 5, 6}
Intersection: {3, 4}
Difference: {1, 2}

Lookups:
Contains 3: true
Lookup 5: null

Best Practices

• Immutable APIs: Use UnmodifiableSetView for public APIs that shouldn't be modified.
• Performance: Prefer UnmodifiableSetView over copying for large Sets.
• Defensive Programming: Wrap Sets in views when passing to untrusted code.
• Documentation: Clearly document when APIs return unmodifiable views.

Source

Dart UnmodifiableSetView Documentation

This tutorial covered Dart's UnmodifiableSetView with practical examples demonstrating its features and usage patterns for immutable Set operations.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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