Why C# Refuses to Compare Collections

C# compares collections by reference rather than by value, and this design decision is intentional. There is no single correct definition of collection equality: two collections could be considered equal if they contain the same elements in the same order (sequence equality), or the same elements regardless of order (content/multiset equality). The post demonstrates implementing custom IEqualityComparer classes — a SequenceComparer using LINQ's SequenceEqual and a ContentComparer that sorts before comparing — to show how developers can define equality themselves. It also highlights performance implications (O(n) for sequence, O(n log n) for content comparison vs. O(1) for reference) and the fundamental problem of defining equality on mutable objects, which can lead to stale or incorrect equality flags after mutation.