Every developer uses for and foreach loops. To iterate a collection, foreach is, in most cases, more convenient than a  for loop. It works with all collection types, including those that are not indexable such as IEnumerable<T>, and doesn't require to access the current element by its index.

But sometimes, we really do need the index of the current item; this usually leads to index-handling manually or switching to a for loop:

// foreach with a "manual" index
int index = 0;
foreach (var item in collection)
{
    DoSomething(item, index);
    index++;
}

// normal for loop
for (int index = 0; index < collection.Count; index++)
{
    var item = collection[index];
    DoSomething(item, index);
}

Couldn't we have the benefits of both foreach and for? It turns out that there's a simple solution, using Linq and tuples. Just write an extension method like this:

using System.Linq;
...

public static IEnumerable<(T item, int index)> WithIndex<T>(this IEnumerable<T> source)
{
    return source.Select((item, index) => (item, index));
}

And now you can do this:

foreach (var (item, index) in collection.WithIndex())
{
    DoSomething(item, index);
}