Counter in foreach loop in C#

It depends what you mean by "it". The iterator knows what index it's reached, yes - in the case of a List<T> or an array. But there's no general index within IEnumerator<T>. Whether it's iterating over an indexed collection or not is up to the implementation. Plenty of collections don't support direct indexing.

(In fact, foreach doesn't always use an iterator at all. If the compile-time type of the collection is an array, the compiler will iterate over it using array[0], array[1] etc. Likewise the collection can have a method called GetEnumerator() which returns a type with the appropriate members, but without any implementation of IEnumerable/IEnumerator in sight.)

Options for maintaining an index:

• Use a for loop
• Use a separate variable

• Use a projection which projects each item to an index/value pair, e.g.

   foreach (var x in list.Select((value, index) => new { value, index }))
   {
       // Use x.value and x.index in here
   }
  

• Use my SmartEnumerable class which is a little bit like the previous option

All but the first of these options will work whether or not the collection is naturally indexed.

Use for instead of foreach. foreach doesn't expose its inner workings, it enumerates anything that is IEnumerable (which doesn't have to have an index at all).

for (int i=0; i<arr.Length; i++)
{
    ...
}

Besides, if what you're trying to do is find the index of a particular item in the list, you don't have to iterate it at all by yourself. Use Array.IndexOf(item) instead.

Your understanding of foreach is incomplete.

It works with any type that exposes IEnumerable (or implements a GetEnumerable method) and uses the returned IEnumerator to iterate over the items in the collection.

How the Enumerator does this (using an index, yield statement or magic) is an implementation detail.

In order to achieve what you want, you should use a for loop:

for (int i = 0; i < mylist.Count; i++)
{
}

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Note:

Getting the number of items in a list is slightly different depending on the type of list

For Collections: Use Count   [property]
For Arrays:      Use Length  [property]
For IEnumerable: Use Count() [Linq method]