C# IFormattable example

Customer.cs

public record Customer(int Id, string FirstName, string LastName, DateOnly BirthDay) : IFormattable
{
    public int Id { get; set; } = Id;
    public string FirstName { get; set; } = FirstName;
    public string LastName { get; set; } = LastName;
    public DateOnly BirthDay { get; set; } = BirthDay;
    public string ToString(string format, IFormatProvider _)
        => format switch
        {
            "F" => $"{Id,-5}{FirstName} {LastName}",
            "N" => $"{FirstName} {LastName}",
            "B" => $"{BirthDay}",
            "I" => $"{Id}",
            _ => $"{Id,-5}{BirthDay} {LastName}, {BirthDay}"
        };
    /// <summary>
    /// Stock version of ToString which returns the full name and age
    /// </summary>
    public override string ToString() 
        => $"{FirstName} {LastName}, age {BirthDay.GetAge()}";

    internal void Deconstruct(out string firstName, out string lastName, out DateOnly birth)
    {
        firstName = FirstName;
        lastName = LastName;
        birth = BirthDay;
    }
}

DateOnlyExtensions.cs

public static class DateOnlyExtensions
{
    public static int GetAge(this DateOnly dateOfBirth)
    {
        var (nYear, nMonth, nDay) = DateTime.Today;

        var a = (nYear * 100 + nMonth) * 100 + nDay;
        var (bYear, bMonth, bDay) = dateOfBirth;
        var b = (bYear * 100 + bMonth) * 100 + bDay;

        return (a - b) / 10000;
    }
}

Program.cs

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

        Customer customer = 
            new(1,"Karen", "Payne", 
                new DateOnly(1956,9,24));
        
        Console.WriteLine($"{customer:F}");
        Console.WriteLine($"{customer:N}");
        Console.WriteLine($"{customer:B}");
        Console.WriteLine($"{customer:I}");
        Console.WriteLine($"{customer}");
        Console.WriteLine($"{customer.ToString()}");

        var (firstName, lastName, birthDate) = customer;

    }

}