public static IEnumerable<KeyValuePair<int, T>> Enumerate<T>(this IEnumerable<T> collection, int startIndex=0)
{
    foreach (var item in collection) { yield return new KeyValuePair<int, T>(startIndex++, item); }
}

Use:
    foreach (var idx_val in Enumerate(Iterable))
    {
        int idx = idx_val.Key;
        var el = idx_val.Value;
    }