EnumerateWithIndex<T>()

static class Extensions {
    public static IEnumerable<(int Index, T Item)> EnumerateWithIndex<T>(this IEnumerable<T> source) {
        int index = 0;
        foreach (var item in source) {
            yield return (index++, item);
        }
    }
}

var l = new List<Employee> {
        new Employee("Fons", 2000),
        new Employee("Jim", 4000),
        new Employee("Ellen", 3000),
    };

foreach (var item in l.EnumerateWithIndex()) {
    Console.WriteLine(item.Index);
    item.Item.RaiseSalary(10);
    Console.WriteLine(item.Item);
}