Concurrent modification during iteration

Why This Happens

In Dart, modifying a list (adding or removing elements) while iterating over it with a for-in loop causes a ConcurrentModificationError. You should iterate over a copy of the list or collect items to remove and process them after the loop.

The Problem

final items = [1, 2, 3, 4, 5];
for (final item in items) {
  if (item.isEven) {
    items.remove(item); // Modifying during iteration!
  }
}

The Fix

final items = [1, 2, 3, 4, 5];
items.removeWhere((item) => item.isEven);

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: Concurrent modification during iteration. This means the collection was changed while being iterated.

2. 2

   Find the cause

   Check for-in loops where add() or remove() is called on the same list being iterated.

3. 3

   Apply the fix

   Use removeWhere(), iterate over a copy with .toList(), or collect changes and apply them after the loop.