Dart UnmodifiableListView

last modified April 4, 2025

In Dart, UnmodifiableListView is an unmodifiable view of a List. It wraps an existing List and prevents modifications through the view.

UnmodifiableListView implements List but throws UnsupportedError for operations that would modify the list. It’s useful for exposing internal lists safely.

Creating an UnmodifiableListView

The simplest way to create an UnmodifiableListView is using its constructor.

main.dart

import ‘dart:collection’;

void main() { var numbers = [1, 2, 3, 4, 5]; var unmodifiableView = UnmodifiableListView(numbers);

print(unmodifiableView);

try { unmodifiableView.add(6); // Will throw } catch (e) { print(‘Error: $e’); } }

We create an unmodifiable view of a mutable list. Attempting to modify it throws an UnsupportedError. The original list can still be modified.

$ dart main.dart [1, 2, 3, 4, 5] Error: Unsupported operation: Cannot add to an unmodifiable list

UnmodifiableListView with List Operations

UnmodifiableListView supports all non-modifying List operations like access and iteration.

main.dart

import ‘dart:collection’;

void main() { var fruits = [‘apple’, ‘banana’, ‘orange’]; var unmodifiableFruits = UnmodifiableListView(fruits);

// Access elements print(‘First fruit: ${unmodifiableFruits[0]}’); print(‘Length: ${unmodifiableFruits.length}’);

// Iteration for (var fruit in unmodifiableFruits) { print(fruit.toUpperCase()); }

// Non-modifying methods print(‘Contains apple: ${unmodifiableFruits.contains(‘apple’)}’); print(‘Index of banana: ${unmodifiableFruits.indexOf(‘banana’)}’); }

We demonstrate read-only operations on an UnmodifiableListView. All standard List access methods work as expected without modification capabilities.

$ dart main.dart First fruit: apple Length: 3 APPLE BANANA ORANGE Contains apple: true Index of banana: 1

Protecting Internal Lists

UnmodifiableListView is commonly used to expose internal lists safely.

main.dart

import ‘dart:collection’;

class Team { final List<String> _members = [];

UnmodifiableListView<String> get members => UnmodifiableListView(_members);

void addMember(String name) { _members.add(name); } }

void main() { var team = Team(); team.addMember(‘Alice’); team.addMember(‘Bob’);

print(‘Team members: ${team.members}’);

try { team.members.add(‘Charlie’); // Will throw } catch (e) { print(‘Cannot modify: $e’); } }

The Team class exposes its members through an UnmodifiableListView. Clients can read but not modify the list directly. Modifications must go through addMember.

$ dart main.dart Team members: [Alice, Bob] Cannot modify: Unsupported operation: Cannot add to an unmodifiable list

UnmodifiableListView vs Unmodifiable List

UnmodifiableListView differs from creating a truly unmodifiable list.

main.dart

import ‘dart:collection’;

void main() { var original = [1, 2, 3]; var view = UnmodifiableListView(original); var unmodifiableCopy = List.unmodifiable(original);

original.add(4); // Modifies the original

print(‘View: $view’); // Reflects changes print(‘Copy: $unmodifiableCopy’); // Doesn’t change

try { view.add(5); // Throws unmodifiableCopy.add(5); // Also throws } catch (e) { print(‘Both prevent modification’); } }

UnmodifiableListView is a live view that reflects changes to the original list. List.unmodifiable creates an independent copy that won’t change.

$ dart main.dart View: [1, 2, 3, 4] Copy: [1, 2, 3] Both prevent modification

Combining with Other Collections

UnmodifiableListView can be combined with other collection operations.

main.dart

import ‘dart:collection’;

void main() { var data = [10, 20, 30, 40, 50];

// Create filtered unmodifiable view var evenView = UnmodifiableListView( data.where((n) => n % 2 == 0).toList());

print(‘Even numbers: $evenView’);

// Create transformed unmodifiable view var squaredView = UnmodifiableListView( data.map((n) => n * n).toList());

print(‘Squared numbers: $squaredView’);

// Attempt modification try { evenView.removeLast(); } catch (e) { print(‘Cannot modify: $e’); } }

We create specialized unmodifiable views by combining filtering and mapping with UnmodifiableListView. The views remain immutable while showing transformed data.

$ dart main.dart Even numbers: [20, 40, 50] Squared numbers: [100, 400, 900, 1600, 2500] Cannot modify: Unsupported operation: Cannot remove from an unmodifiable list

Best Practices

• Defensive Programming: Use to prevent accidental modifications.

• Performance: Prefer over copying large lists when possible.

• Documentation: Clearly indicate when APIs return unmodifiable views.

• Alternatives: Consider List.unmodifiable for truly immutable snapshots.

Source

Dart UnmodifiableListView Documentation

This tutorial covered Dart’s UnmodifiableListView with practical examples demonstrating its key features and usage patterns.

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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