C# 6.0 Features: FormattableString on .Net 4.0

Program.cs

using System;
using System.Globalization;

static class Program
{
	private static void Main()
	{
		double d = 3.14;
		IFormattable s = $"{d}";
		Console.WriteLine(s.ToString(null, CultureInfo.GetCultureInfo("cs-cz")));
		Console.WriteLine(s.ToString(null, CultureInfo.InvariantCulture));
	}
}

namespace System.Runtime.CompilerServices
{
	static class FormattableStringFactory
	{
		public static FormattableString Create(string s, params object[] args)
		{
			return new FormattableString(s, args);
		}
	}

	class FormattableString : IFormattable
	{
		private readonly string s;
		private readonly object[] args;

		public FormattableString(string s, object[] args)
		{
			this.s = s;
			this.args = args;
		}

		public string ToString(string ignored, IFormatProvider formatProvider)
		{
			return string.Format(formatProvider, s, args);
		}
	}
}