.NET Performance: Frozen vs. Immutable Collections — Which One is Faster?

Hossein Kohzadi

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Jul 25, 2024

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As developers, we often find ourselves grappling with choices between different data structures and collections to best suit our applications’ needs. In the latest versions of .NET Core, two options for collections that have garnered significant attention are Frozen Collections and Immutable Collections. Both offer unique benefits and cater to different use cases. Let’s dive into these collections, understand their differences, and see where each one shines.

Immutable Collections: A Quick Recap

Immutable collections, as the name suggests, are collections that cannot be modified once created. Any operation that attempts to modify the collection results in the creation of a new collection with the desired changes, leaving the original collection unchanged. This immutability offers several advantages:

1. Thread Safety: Immutable collections are inherently thread-safe since their state cannot change after creation. This makes them ideal for multi-threaded environments where synchronization issues might arise.
2. Predictability: With immutable collections, you can be sure that the data will remain constant, providing predictability in your codebase. This immutability simplifies reasoning about the code and reduces bugs related to unexpected state changes.
3. Functional Programming: Immutable collections align well with functional programming paradigms, where immutability is a core principle. They facilitate the creation of pure functions and support declarative programming styles.
4. Optimized Implementations: As highlighted by Stephen Toub, implementations of immutable collections are optimized for specific use cases. While methods like Add and Remove create new instances derived from the original, operations like TryGetValue are optimized for fast reading, making them efficient for many scenarios.

The latest .NET Core versions come with a robust set of immutable collections available in the System.Collections.Immutable namespace, including ImmutableList, ImmutableDictionary, ImmutableHashSet, and more.

Frozen Collections: A New Player in the Field

Frozen collections are a relatively new addition to .NET Core, introduced to provide a more optimized and memory-efficient alternative to immutable collections. The key characteristic of frozen collections is that they start as mutable, allowing you to build them dynamically. Once you are done adding or modifying elements, you can freeze the collection, making it immutable from that point forward.

Here’s why frozen collections can be a game-changer:

1. Performance: Frozen collections can offer better performance compared to immutable collections, especially during the building phase. Since they are mutable during this phase, modifications are less expensive. Once frozen, they provide similar benefits to immutable collections, such as thread safety and predictability.
2. Memory Efficiency: Frozen collections are designed to be more memory-efficient. By freezing a collection, .NET can optimize its memory layout, reducing overhead and potentially improving application performance.
3. Ease of Use: Frozen collections provide a more flexible approach to immutability. You can work with a mutable collection until you reach a stable state, then freeze it. This can simplify code where a collection is built incrementally but should remain unchanged once finalized.
4. Optimized for Reads: Stephen Toub notes that frozen collections make reads faster for all subsequent operations. By spending more time at construction to put the data into an optimized form, operations like TryGetValue become very fast, making frozen collections suitable for scenarios where read performance is critical.

When to Use Which?

The choice between frozen and immutable collections largely depends on the specific needs of your application:

• Immutable Collections: Use these when you need guaranteed immutability throughout the lifecycle of the collection. They are ideal for functional programming, multi-threaded environments, and scenarios where predictability and thread safety are paramount.
• Frozen Collections: Opt for these when you have a clear phase distinction between building and using the collection. They are particularly useful when performance during the build phase is a concern, and you want to leverage the memory efficiency and thread safety of an immutable collection once the build phase is complete.

Conclusion

Both frozen and immutable collections offer significant advantages, each catering to different scenarios in modern .NET Core applications. Immutable collections provide simplicity, and thread safety, and align with functional programming principles, making them a solid choice for many applications. Frozen collections bring performance and memory efficiency to the table, especially when collections are built dynamically and then remain unchanged.

Understanding the strengths and appropriate use cases for each collection type can help you make informed decisions, optimizing both the performance and reliability of your applications. As .NET Core continues to evolve, embracing these collection types will undoubtedly lead to more robust and maintainable codebases.