C# FormattableStringExtensions.cs

FormattableStringExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;

    public static class FormattableStringExtensions
    {
        public static FormattableString AsFormattable(this string str) {
            return FormattableStringFactory.Create(str);
        }

        class FormattablePlaceholder : IFormattable
        {
            public Func<string, IFormatProvider, string> ValueByFormat { get; set; }
            public string ToString(string format, IFormatProvider formatProvider)
            {
                return ValueByFormat(format, formatProvider);
            }
        }


        public static FormattableString Join(string separator, params FormattableString[] strings)
        {
            var iArg = 0;
            var formats = strings.Select(str =>
            {
                var args = Enumerable.Range(iArg, str.ArgumentCount).ToArray();
                iArg += str.ArgumentCount;
                return string.Format(str.Format, args: args.Select(arg=> new FormattablePlaceholder() {
                    ValueByFormat = (fmt, fp) => fmt != null ? $"{{{arg}}}:{fmt}}}" : $"{{{arg}}}"
                }).ToArray());
            }).ToArray();

            return FormattableStringFactory.Create(string.Join(separator, value:formats), arguments: strings.SelectMany(s => s.GetArguments()).ToArray());
        }

        public static FormattableString Add(this FormattableString formattable, params FormattableString[] strings)
        {
            strings = new[] { formattable }.Concat(strings).ToArray();
            return Join(string.Empty,strings);
        }
    }