Question

How can you best explain why replacing an element of a collection you are looping using foreach is permited if you make a copy of it. Example:

foreach(Item item in Items)
{
   item.modify //or remove or add
}
// will not work

foreach(Item item in Items.ToList())
{
   item.modify //or remove. or add
}

//will work altough i dont get it because i am now iterating trough the temporary list
//and changing its elements. 
//In my understanding its not like im iterating the list(.ToList) and modifying the source items
//(Items). A graphic representation would be welcome, my interest is to understand the
//matter logically

Answer 1

The Best answer would be that List has some kind of Tracking over its list items and can update its items as you ask but a simple IEnumerable does not so it will not allow you to change them.