Code inspection: Possible '{0}.{1}: Collection was modified'

This inspection reports a foreach loop that modifies the same collection it is enumerating. That can throw InvalidOperationException at runtime with a message like "Collection was modified".

Example

                foreach (var item in items)
                {
                    items.Add(item);
                }
            

                foreach (var item in items.ToList())
                {
                    items.Add(item);
                }
            

Quick-fix

The quick-fix copies the elements before enumeration so the loop iterates over a stable snapshot.

01 April 2026