Dart UnmodifiableMapView

last modified April 4, 2025

In Dart, UnmodifiableMapView is an unmodifiable view of a Map. It provides read-only access to an existing Map while preventing modifications.

UnmodifiableMapView implements the Map interface but throws exceptions for modification operations. It's useful for exposing internal maps safely.

Creating an UnmodifiableMapView

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

import 'dart:collection';

void main() {
  var originalMap = {'a': 1, 'b': 2, 'c': 3};
  var unmodifiableView = UnmodifiableMapView(originalMap);

  print(unmodifiableView);
  
  try {
    unmodifiableView['d'] = 4; // Throws
  } catch (e) {
    print('Error: $e');
  }
}

We create an unmodifiable view of a regular map. Attempting to modify it throws an UnsupportedError. The view reflects changes to the original map.

$ dart main.dart
{a: 1, b: 2, c: 3}
Error: Unsupported operation: Cannot modify unmodifiable map

UnmodifiableMapView with Map Methods

UnmodifiableMapView supports all read operations from the Map interface.

import 'dart:collection';

void main() {
  var scores = {'Alice': 95, 'Bob': 87, 'Charlie': 92};
  var view = UnmodifiableMapView(scores);

  print('Alice\'s score: ${view['Alice']}');
  print('Contains Bob? ${view.containsKey('Bob')}');
  print('Keys: ${view.keys}');
  print('Values: ${view.values}');
  print('Length: ${view.length}');
}

We demonstrate various read operations on an UnmodifiableMapView. All these operations work the same as with regular maps but are safe from modifications.

$ dart main.dart
Alice's score: 95
Contains Bob? true
Keys: (Alice, Bob, Charlie)
Values: (95, 87, 92)
Length: 3

Dynamic Updates from Original Map

UnmodifiableMapView reflects changes made to the original underlying map.

import 'dart:collection';

void main() {
  var original = {'x': 10, 'y': 20};
  var view = UnmodifiableMapView(original);

  print('Initial view: $view');
  
  original['z'] = 30;
  print('After original modification: $view');
  
  original.remove('x');
  print('After original removal: $view');
}

Changes to the original map are immediately visible in the view. The view itself remains unmodifiable while the original can still be changed.

$ dart main.dart
Initial view: {x: 10, y: 20}
After original modification: {x: 10, y: 20, z: 30}
After original removal: {y: 20, z: 30}

Iterating Over UnmodifiableMapView

UnmodifiableMapView supports all standard map iteration methods.

import 'dart:collection';

void main() {
  var colors = {'red': '#FF0000', 'green': '#00FF00', 'blue': '#0000FF'};
  var unmodifiableColors = UnmodifiableMapView(colors);

  print('Keys:');
  for (var key in unmodifiableColors.keys) {
    print(key);
  }

  print('\nValues:');
  for (var value in unmodifiableColors.values) {
    print(value);
  }

  print('\nEntries:');
  unmodifiableColors.forEach((key, value) => print('$key: $value'));
}

We demonstrate three ways to iterate through an UnmodifiableMapView. The view maintains the same iteration order as the original map.

$ dart main.dart
Keys:
red
green
blue

Values:
#FF0000
#00FF00
#0000FF

Entries:
red: #FF0000
green: #00FF00
blue: #0000FF

UnmodifiableMapView with Custom Objects

UnmodifiableMapView works with custom objects just like regular maps.

import 'dart:collection';

class Product {
  final String id;
  final String name;
  
  Product(this.id, this.name);
  
  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is Product && id == other.id && name == other.name;
  
  @override
  int get hashCode => id.hashCode ^ name.hashCode;
}

void main() {
  var products = {
    Product('p1', 'Laptop'): 999,
    Product('p2', 'Phone'): 699,
  };
  
  var unmodifiableProducts = UnmodifiableMapView(products);
  
  var searchKey = Product('p1', 'Laptop');
  print('Laptop price: \$${unmodifiableProducts[searchKey]}');
  
  try {
    unmodifiableProducts[Product('p3', 'Tablet')] = 299;
  } catch (e) {
    print('Error: $e');
  }
}

We use custom Product objects as keys in an UnmodifiableMapView. The view correctly handles key lookups while preventing modifications.

$ dart main.dart
Laptop price: $999
Error: Unsupported operation: Cannot modify unmodifiable map

Best Practices

• Immutable Exposure: Use to safely expose internal maps.
• Performance: No overhead compared to original map.
• Documentation: Clearly document returned views as unmodifiable.
• Null Safety: Works well with non-nullable types.

Source

Dart UnmodifiableMapView Documentation

This tutorial covered Dart's UnmodifiableMapView 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.

List all Dart tutorials.