How do you get the index of the current iteration

Question

Is there some rare language construct I haven't encountered (like the few I've learned recently, some on Stack Overflow) in C# to get a value representing the current iteration of a foreach loop?

For instance, I currently do something like this depending on the circumstances:

int i=0;
foreach (Object o in collection)
{
    // ...
    i++;
}

Answer 1

For interest, Phil Haack just wrote an example of this in the context of a Razor Templated Delegate (http://haacked.com/archive/2011/04/14/a-better-razor-foreach-loop.aspx)

Effectively he writes an extension method which wraps the iteration in an "IteratedItem" class (see below) allowing access to the index as well as the element during iteration.

public class IndexedItem<TModel> {
  public IndexedItem(int index, TModel item) {
    Index = index;
    Item = item;
  }

  public int Index { get; private set; }
  public TModel Item { get; private set; }
}

However, while this would be fine in a non-Razor environment if you are doing a single operation (i.e. one that could be provided as a lambda) it's not going to be a solid replacement of the for/foreach syntax in non-Razor contexts.

Answer 2

If the collection is a list, you can use List.IndexOf, as in:

foreach (Object o in collection)
{
    // ...
    @collection.IndexOf(o)
}

Answer 3

I wasn't sure what you were trying to do with the index information based on the question. However, in C#, you can usually adapt the IEnumerable.Select method to get the index out of whatever you want. For instance, I might use something like this for whether a value is odd or even.

string[] names = { "one", "two", "three" };
var oddOrEvenByName = names
    .Select((name, index) => new KeyValuePair<string, int>(name, index % 2))
    .ToDictionary(kvp => kvp.Key, kvp => kvp.Value);

This would give you a dictionary by name of whether the item was odd (1) or even (0) in the list.

Answer 4

I built this in LINQPad:

var listOfNames = new List<string>(){"John","Steve","Anna","Chris"};

var listCount = listOfNames.Count;

var NamesWithCommas = string.Empty;

foreach (var element in listOfNames)
{
    NamesWithCommas += element;
    if(listOfNames.IndexOf(element) != listCount -1)
    {
        NamesWithCommas += ", ";
    }
}

NamesWithCommas.Dump();  //LINQPad method to write to console.

You could also just use string.join:

var joinResult = string.Join(",", listOfNames);

Answer 5

Ian Mercer posted a similar solution as this on Phil Haack's blog:

foreach (var item in Model.Select((value, i) => new { i, value }))
{
    var value = item.value;
    var index = item.i;
}

This gets you the item (item.value) and its index (item.i) by using this overload of Linq's Select:

the second parameter of the function [inside Select] represents the index of the source element.

The new { i, value } is creating a new anonymous object.

Answer 6

Finally C#7 has a decent syntax for getting an index inside of a foreach loop (i. e. tuples):

foreach (var (item, index) in collection.WithIndex())
{
    Debug.WriteLine($"{index}: {item}");
}

A little extension method would be needed:

public static IEnumerable<(T item, int index)> WithIndex<T>(this IEnumerable<T> self)       
   => self.Select((item, index) => (item, index));