What causes modifying a collection while iterating?

Adding to or removing from a collection while a foreach loop iterates it invalidates the iterator, throwing an exception or skipping and double-processing elements.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix a Crash from Modifying a Collection While Iterating

Quick answer: Iterate over a copy when you need to modify, collect changes and apply them after the loop, or use an index-based reverse loop for removals.

Changing a collection while looping over it is a classic crash. Iterating a copy or deferring the changes fixes it. Here is how.

How to fix it

1. Iterate a copy

If you must add or remove during the loop, iterate over a copy of the collection and modify the original. The copy is not invalidated by changes to the source.

2. Defer the modifications

Collect the items to add or remove into a separate list during the loop, then apply them after it finishes. This keeps the iterated collection stable while you walk it.

3. Use a reverse index loop for removals

Removing while iterating forward shifts indices and skips elements. A reverse for loop (from the end to the start) lets you remove safely because earlier indices are unaffected.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every your game error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

The errors you never hear about are the ones quietly costing you players. Visibility turns them into a worklist.