NET 8 introduces a new type called FrozenSet<T>

.NET 8 presents an array of performance enhancements that developers can leverage without modifying their existing code. By simply targeting net8.0 in the project file, one can reap the benefits of these improvements. Nonetheless, to utilize specific advanced features such as frozen collections, comprehension and code adaptation are required. This article will delve into this novel feature and examine its potential to optimize .NET applications.

Understanding Read-Only and Immutable Collections

Since its inception, the .NET framework has offered a wide range of collection types such as lists, queues, stacks, and dictionaries. These collections provide greater flexibility compared to arrays, as they allow items to be added or removed dynamically, making them suitable for a variety of programming scenarios. However, there are instances where a collection needs to be populated only once and remain unchanged throughout the application's lifecycle, or perhaps only a specific part of your code should have the ability to modify it. This is where read-only collections come into play, offering a valuable solution for these particular use cases.

Read-only collections are designed to provide a level of immutability, ensuring that the contents of the collection cannot be altered once they have been initialized. This can be particularly useful in situations where data integrity is of utmost importance, as it prevents accidental or unauthorized modifications to the collection's contents. Additionally, read-only collections can help improve the performance of your application by reducing the overhead associated with managing dynamic collections, as well as providing a level of thread safety in multithreaded environments.

In the .NET framework, read-only collections are implemented through a set of specialized classes, such as ReadOnlyCollection<T>, ReadOnlyDictionary<TKey, TValue>, and ReadOnlyObservableCollection<T>. These classes essentially wrap around their mutable counterparts, exposing a read-only view of the underlying collection. This allows developers to create collections that can be safely shared across different parts of the application without the risk of unintended modifications.

Introducing Frozen Collections in .NET 8

.NET 8 introduces a groundbreaking concept: frozen collections. What sets them apart from immutable collections?

Performance Optimization

Generic collections, by their nature, present performance challenges. Their ability to handle any type means the .NET team cannot optimize algorithms for specific types like string or int, which are commonly used. Additionally, the dynamic nature of these collections, allowing additions and removals, restricts potential optimizations.

Frozen collections tackle these issues. The underlying concept is that most collections, once populated, seldom change. If these collections are optimized at the creation stage, it results in initial overhead but significantly enhances read performance afterward.

FrozenDictionary<TKey, TValue> and FrozenSet<T>

.NET 8 introduces two frozen collections: FrozenDictionary<TKey, TValue> and FrozenSet<T>. These collections are designed for the most common scenarios where the frozen concept offers significant advantages. Once a frozen collection is created, it cannot be modified, allowing the .NET runtime to apply optimizations that were previously unattainable with dynamic collections.


In conclusion, .NET 8 introduces advanced features like frozen collections, which require understanding and code adaptation. While read-only collections have provided a degree of immutability, frozen collections offer a new level of performance optimization. They address the challenge of generic collections and take advantage of the fact that most collections, once populated, rarely change. The newly introduced FrozenDictionary<TKey, TValue> and FrozenSet<T> are designed for common scenarios where the frozen concept can provide significant benefits. By preventing modifications once a frozen collection is created, the .NET runtime can apply optimizations that were previously unachievable with dynamic collections.

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