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collections

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2026-11-07 errors dotnet csharp

Fix: Collection was modified; enumeration operation may not execute in C#

You changed a List or Dictionary while a foreach was walking it. Iterate a copy, use RemoveAll, loop backwards with for, or lock both sides. Which mutations throw on .NET 10 and 11, and why Dictionary.Remove does not.

2026-10-25 dart flutter collections

How to remove items from a Dart List while iterating over it without a ConcurrentModificationError

Calling remove inside a for-in loop or forEach throws ConcurrentModificationError. Use removeWhere for in-place filtering, build a new list with where or a collection-for, or loop backwards by index. Verified on Dart 3.13.7, with a benchmark of why removeAt in a loop is the slow option.

2026-09-12 comparison csharp dotnet

Lookup<TKey, TElement> vs Dictionary<TKey, List<TValue>> for duplicate keys in C#

Use ToLookup when you group once and only read: it is immutable, returns an empty sequence for missing keys, accepts null keys and keeps first-seen order. Use Dictionary<TKey, List<TValue>> when the groups change after construction or cross a JSON boundary.