Vanara/Core/CompositeFormatter.cs

using System.Collections.Generic;
using System.Globalization;

namespace Vanara;

// Leveraged code from t3chb0t and Bogdan Yarema at https://codereview.stackexchange.com/questions/138747/custom-string-formatters

/// <summary>Extension method to combine formatter instances.</summary>
public static class FormatterComposer
{
	/// <summary>Adds a chain of formatters with specific cultures to make a composite.</summary>
	/// <typeparam name="T">A <see cref="Formatter"/> derived type.</typeparam>
	/// <param name="formatter">The formatter instance to start the chain.</param>
	/// <param name="culture">The culture.</param>
	/// <returns>A composite formatter.</returns>
	/// <example>
	/// <code lang="cs">
	/// // Build composite formatter from custom formatters derived from Formatter
	/// var formatter = Formatter.Default().Add&lt;CustomFormatter1&gt;().Add&lt;CustomFormatter2&gt;();
	/// // Use custom format extensions defined in the custom formatters to format the string
	/// var output = string.Format(formatter, "{0:cf1} = {0:cf2}", 512);
	/// </code>
	/// </example>
	public static Formatter Add<T>(this Formatter formatter, CultureInfo? culture = null) where T : Formatter, new()
	{
		var newFormatter = new T();
		if (formatter is not CompositeFormatter compositeFormatter)
			return new CompositeFormatter(culture, formatter, newFormatter);
		compositeFormatter.Add(newFormatter);
		return compositeFormatter;
	}
}

/// <summary>Binds multiple formatters together.</summary>
/// <seealso cref="Formatter"/>
internal sealed class CompositeFormatter : Formatter
{
	private readonly List<Formatter> _formatters;

	/// <summary>Initializes a new instance of the <see cref="CompositeFormatter"/> class.</summary>
	/// <param name="culture">The culture.</param>
	/// <param name="formatters">The formatters.</param>
	public CompositeFormatter(CultureInfo? culture = null, params Formatter[] formatters) : base(culture) => _formatters = new(formatters);

	/// <summary>Adds the specified formatter.</summary>
	/// <param name="formatter">The formatter.</param>
	public void Add(Formatter formatter) => _formatters.Add(formatter);

	/// <summary>
	/// Converts the value of a specified object to an equivalent string representation using specified format and culture-specific
	/// formatting information.
	/// </summary>
	/// <param name="format">A format string containing formatting specifications.</param>
	/// <param name="arg">An object to format.</param>
	/// <param name="formatProvider">An object that supplies format information about the current instance.</param>
	/// <returns>
	/// The string representation of the value of <paramref name="arg"/>, formatted as specified by <paramref name="format"/> and <paramref name="formatProvider"/>.
	/// </returns>
	public override string Format(string? format, object? arg, IFormatProvider? formatProvider)
	{
		foreach (var formatter in _formatters)
		{
			var result = formatter.Format(format, arg, formatProvider);
			if (result != null)
				return result;
		}
		return HandleOtherFormats(format, arg);
	}
}

/// <summary>Base class for expandable formatters.</summary>
public abstract class Formatter : IFormatProvider, ICustomFormatter
{
	/// <summary>Initializes a new instance of the <see cref="Formatter"/> class.</summary>
	/// <param name="culture">The culture.</param>
	protected Formatter(CultureInfo? culture = null) => Culture = culture ?? CultureInfo.InvariantCulture;

	/// <summary>Gets the culture.</summary>
	/// <value>The culture.</value>
	public CultureInfo Culture { get; }

	/// <summary>Gets a default instance of a composite formatter.</summary>
	/// <param name="culture">The culture.</param>
	/// <returns>A composite formatter.</returns>
	public static Formatter Default(CultureInfo? culture = null) => new CompositeFormatter(culture);

	/// <summary>
	/// Converts the value of a specified object to an equivalent string representation using specified format and culture-specific
	/// formatting information.
	/// </summary>
	/// <param name="format">A format string containing formatting specifications.</param>
	/// <param name="arg">An object to format.</param>
	/// <param name="formatProvider">An object that supplies format information about the current instance.</param>
	/// <returns>
	/// The string representation of the value of <paramref name="arg"/>, formatted as specified by <paramref name="format"/> and <paramref name="formatProvider"/>.
	/// </returns>
	public abstract string Format(string? format, object? arg, IFormatProvider? formatProvider);

	/// <summary>Returns an object that provides formatting services for the specified type.</summary>
	/// <param name="formatType">An object that specifies the type of format object to return.</param>
	/// <returns>
	/// An instance of the object specified by <paramref name="formatType"/>, if the IFormatProvider implementation can supply that type of
	/// object; otherwise, <see langword="null"/>.
	/// </returns>
	public virtual object? GetFormat(Type? formatType) => formatType == typeof(ICustomFormatter) ? this : null;

	/// <summary>Helper method that can be used inside the Format method to handle unrecognized formats.</summary>
	/// <param name="format">A format string containing formatting specifications.</param>
	/// <param name="arg">An object to format.</param>
	/// <returns>The string representation of the value of <paramref name="arg"/>, formatted as specified by <paramref name="format"/>.</returns>
	protected string HandleOtherFormats(string? format, object? arg) => (arg as IFormattable)?.ToString(format, Culture) ?? arg?.ToString() ?? string.Empty;
}