You can't modify a collection while iterating over it, so try this instead.
Where items is IList, item is an Object T, and you want to remove based on the item's ItemID:
if (items != null)
{
for (int j = items.Count - 1; j >= 0; j--)
{
if (items[j].ItemID == item.ItemID)
{
items.Remove(items[j]);
}
}
}