FormattableString on .NET 4.0

FormattableString.cs

namespace System
{
    using System.Globalization;

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

        string IFormattable.ToString(string ignored, IFormatProvider formatProvider) => this.ToString(formatProvider);

        public override string ToString() => this.ToString(CultureInfo.CurrentCulture);

        public static string Invariant(FormattableString formattable)
        {
            if (formattable == null)
            {
                throw new ArgumentNullException(nameof(formattable));
            }

            return formattable.ToString(CultureInfo.InvariantCulture);
        }
    }
}

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