InvalidOperationException: Collection was modified#

List addition#

Use a reverse for loop. Because the loop is done in reverse, the elements added to the end are not iterated when you add them, because the index approaches 0.

for (int i = list.Count - 1; i >= 0; i--)
{
    if (condition)
        list.Add(newItem);
}

You can perform the above logic with a forward loop, but you must correctly manage the counter to avoid infinite loops. Caching the collection count instead of evaluating it in the for would prevent it from iterating over the newly added elements.

You can autocomplete for to get a correctly written for loop, and forr for a reverse loop.

I cannot use a for loop here.