InvalidOperationException after removing an element in an arrayList [duplicate]

Answer 1

Try iterating over it this way.

for(int i = 0; i < list.Count(); i++){
   if ((list[i] > 2) && (list[i] % 2 == 0)) {
                list.RemoveAt(i); 
                i--; //as offsets have shifted by one due to removal
            }
}

You are no longer iterating over the list. So this should work.

Answer 2

I wouldn't even bother removing elements. Just return the elements you do want as a new list:

List<int> RemoveDivTwo(List<int> list) {
    return list.Where(i => i % 2 == 1 || i <= 2).ToList();
}

Answer 3

The exception is thrown because, foreach loop calls collectionName.GetEnumerator method before it starts iterating through the list of items. GetEnumerator is not called again unless you start a new foreach loop on the collection. The list cannot be modified inside the foreach loop, it is meant only for read-only operations on the list.

You may use for loop for iterating as well as modifying the elements from the list.