Immutable Collections

In software development, protecting data from unintended modifications is critical for building reliable, bug-free applications. While C# provides features to restrict how standard collections are accessed, these tools do not completely freeze the underlying data. To achieve absolute certainty that a dataset remains constant across multiple threads or method calls, we must explore true immutability.

The limitation of read-only interfaces

In a previous lesson, we used the IReadOnlyList<T> interface to expose a collection safely. This interface prevents external callers from using methods like Add() or Remove(). However, IReadOnlyList<T> is simply a restrictive wrapper around a mutable collection. It does not make the underlying data truly unchangeable.

If a method retains a reference to the original List<T>, it can still modify the data. The read-only view will immediately reflect those changes, and this behavior can lead to unpredictable bugs in complex or multithreaded applications.