c# - Why can't you modify a collection in a for each loop

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

Because Enumerator relays on count of the elements in collection and you're not premitted to change it during the iteration.

If you make a copy of the list (responding to your question) you iterating over a copy of the collection you're changing. That is.