NET9 Enumerable.Index<T>

[kareninstructor]

Enumerable.Index<T> returns an enumerable that incorporates the element's index into a tuple.

using static System.Globalization.DateTimeFormatInfo;

internal partial class Program
{
    static void Main(string[] args)
    {

        foreach (var (month, index) in Months.Index())
        {
            Console.WriteLine($"{index,-5}{month}");
        }

        Console.ReadLine();

    }

    public static List<string> Months => CurrentInfo.MonthNames[..^1].ToList();
}

Some developers may not care for the above, then they might use.

using static System.Globalization.DateTimeFormatInfo;
internal partial class Program
{
    static void Main(string[] args)
    {
        var index = -1;
        foreach (var month in Months)
        {
            ++index;
            Console.WriteLine($"{index,-5}{month}");
        }

        Console.ReadLine();

    }

    public static List<string> Months => CurrentInfo.MonthNames[..^1].ToList();
}

For the developer targeting a .NET Core Framework prior to NET 9 the following is similar to NET 9 as per the first example.

public static IEnumerable<(int Index, TSource Item)> Index<TSource>(this IEnumerable<TSource> source)
{
    int index = -1;
    foreach (TSource element in source)
    {
        checked
        {
            index++;
        }

        yield return (index, element);
    }
}