IFormattable in .NET 8

IFormattableMoney.cs

using System.Collections.Concurrent;
using System.Globalization;

var money = new Money(1234.56m, "USD");
Console.WriteLine(money.ToString("N", new CultureInfo("en-EN"))); // 1,234.56 USD

money = new Money(1234.56m, "EUR");
Console.WriteLine(money.ToString("N", new CultureInfo("de-DE"))); // 1.234,56 EUR

money = new Money(1234.56m, "JPY");
Console.WriteLine(money.ToString("N", new CultureInfo("ja-JP"))); // 1,234.56 JPY

money = new Money(1234.56m, "GBP");
Console.WriteLine(money.ToString("N", new CultureInfo("en-GB"))); // 1,234.56 GBP

public sealed class MoneyFormatter
{
    private static  ConcurrentDictionary<string, string> CurrencySymbolsCache { get; } = new();

    public static string Format(Money money, string? format, IFormatProvider? formatProvider)
    {
        formatProvider ??= CultureInfo.CurrentCulture;
        format = EnsureValidFormat(format);

        return format.ToUpperInvariant() switch
        {
            "N" => FormatAsNumber(money, formatProvider),
            "C" => FormatAsCurrency(money, formatProvider),
            _ => throw new FormatException($"The {format} format string is not supported.")
        };
    }

    private static string FormatAsNumber(Money money, IFormatProvider formatProvider) =>
        $"{money.Amount.ToString("N2", formatProvider)} {money.CurrencyCode}";

    private static string FormatAsCurrency(Money money, IFormatProvider formatProvider) =>
        $"{GetCurrencySymbol(money.CurrencyCode, formatProvider)}{money.Amount.ToString("N2", formatProvider)}";

    private static string EnsureValidFormat(string? format) => 
        string.IsNullOrWhiteSpace(format) ? "N" : format.Trim();

    private static string GetCurrencySymbol(string currencyCode, IFormatProvider formatProvider) =>
        CurrencySymbolsCache.GetOrAdd(currencyCode, code => LookupCurrencySymbol(currencyCode, formatProvider));

    private static string LookupCurrencySymbol(string currencyCode, IFormatProvider formatProvider)
    {
        if (formatProvider is CultureInfo cultureInfo && !string.IsNullOrEmpty(cultureInfo.Name))
        {
            var region = new RegionInfo(cultureInfo.Name);
            if (region.ISOCurrencySymbol == currencyCode)
            {
                return region.CurrencySymbol;
            }
        }
        return $"[{currencyCode}]";
    }
}

public record Money(decimal Amount, string CurrencyCode) : IFormattable
{
    private static MoneyFormatter Formatter { get; } = new();

    public string ToString(string? format, IFormatProvider? formatProvider) =>
        MoneyFormatter.Format(this, format, formatProvider);
}

Here's a breakdown of the code.

Purpose:

• This code neatly formats money values (amounts) based on different currencies and cultural preferences.
• It makes it easy to display amounts like 1234.56 as "$1,234.56" (USD) or "1.234,56 €" (EUR).

Key Parts:

1. The Money Record:

   • Defines a simple way to store money data:
     • Amount: The decimal value (e.g., 1234.56).
     • CurrencyCode: The currency (e.g., "USD", "EUR", "JPY").

2. MoneyFormatter Class:

   • Handles the actual formatting of Money.
   • Format Method: The core function that formats based on a format code:
     • "N" - Number format: "1,234.56 USD"
     • "C" - Currency format: "$1,234.56"
   • Uses a dictionary (CurrencySymbolsCache) to efficiently store currency symbols.

How it Works (Example):

var money = new Money(1234.56m, "USD");
Console.WriteLine(money.ToString("C", new CultureInfo("en-US"))); // Output: $1,234.56

• Creation: We create a Money object representing $1234.56.
• Formatting: We call money.ToString to format it, specifying:
  • "C" to indicate currency format.
  • "en-US" culture for US-specific symbols and styles.