Dart Immutable Collections

September 29, 2022

Dart makes it pretty easy to create immutable classes, and several patterns encourage you to use them. However, we often don't use immutable collections correctly. If a class has a collection property that is not immutable, the class is not actually immutable. It's possible to add, remove or change one of the collection elements, which means we can mutate the overall class. For a class to be immutable, all members must be immutable, so collections must also be immutable.

What is an Immutable Collection?

An immutable collection is a collection that you can't change. You cannot add, remove, modify the elements, or reorder the elements. The normal constructor of List<T> type does not qualify as immutable because it allows you to do all these things. For example, this class is not actually immutable because we can change elements in the strings list.

It is impossible to modify an immutable collection. Still, the collection in this example is mutable, which makes class A mutable even though we marked it with the @immutable annotation.

How Can We Use Immutable Collections?

An immutable collection should have runtime safety and compile time safety. That means a running app cannot modify the collection, and the app should not compile if we try to modify the list. There are a few packages on Pub.dev that do this, but I'm going to point you to the fixed_collections package because it has an important feature. 

The collections in this package are normal Lists, Sets and Maps. FixedList implements the List interface, FixedSet implements the Set interface, and FixedMap implements the Map interface. You can pass them into any function or constructor that accepts those basic collections. You don't have to convert between collection types.

Print Length

Under the hood, these collections just use the unmodifiable constructors on their class. They don't have any extra logic, but if you use the mutable members with code analysis errors turned on, you will see errors at compile time. You could use these constructors, but the unmodifiable constructor doesn't give you any compilation time errors. In fact, these constructors, without the benefit of compile-time errors, may make your code more error-prone because you may accidentally modify the collection at runtime and cause an error. Here is an example with compile time errors.

Compile Time Error Example

In contrast, other immutable collections take a different approach. The built_collection package uses the builder pattern to create built collections. It's a good package, but the types don't implement the common interfaces like List<>. This code doesn't compile because you can't pass a BuiltList to a function that accepts List.

Built List Conversion Error

Note: you can convert a BuiltList to a List, but you have to call toList()

To use the fixed_collections package, install it, and turn on errors in your analysis options file. This is a simple example. You can use these collections like any other List, Set or Map.

Equality (Comparing Lists)

You may need to override the == operator or hashCode on your class. But, as this linter rule says:

When you define ==, you also have to define hashCode. Both of those should take into account the object's fields. If those fields change then that implies the object's hash code can change. Most hash-based collections don't anticipate that—they assume an object's hash code will be the same forever and may behave unpredictably if that isn't true.

So, if your class has a mutable collection and the class's hash code depends on the collection's hash code, it may behave unpredictably. This is one strong reason that we should use immutable collections on immutable classes. 

fixed_collections doesn't override the equality behaviour of your collection. Other packages do this, and this is not the library's focus. Take a look at listEquals for basic list comparisons, and consider using the equatable package if you need to override equality in your class. The library documentation says this:

Equatable is designed to only work with immutable objects, so all member variables must be final (This is not just a feature of Equatable - overriding a hashCode with a mutable value can break hash-based collections)

Again, it's important to use immutable collections on your immutable classes. Here is an example of using FixedList with equatable. This prints true.

Maximizing Immutability

We allow opportunities to accept mutable lists if we accept the List<> type in Dart or Flutter. Use FixedList, FixedSet, and FixedMap directly in your code, so it's harder to receive mutable lists. Change your classes and functions to accept the fixed version of the type. Here is a Flutter example.


Use immutable lists where you can. They make your code less error-prone. There are several packages on pub dev that you can experiment with, but you want to make sure that whichever solution you run with, you see errors at compile time. 

Photo by Pixabay from Pexels

My photoChristian Findlay

I am a Flutter, .NET and Cloud Developer living in Melbourne, Australia. I've been developing software and mentoring teams for over twenty years.

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