using System;

using System.Globalization;

using System.Runtime.CompilerServices;

​

public class Program

{

    public sealed class ExampleCustomFormatter: IFormatProvider, ICustomFormatter

    {

        public object? GetFormat(Type? formatType) => formatType == typeof(ICustomFormatter) ? this : null;

        public string Format(string? format, object? arg, IFormatProvider? formatProvider) => format == "S" && arg is byte[] i ? Convert.ToBase64String(i) : arg is IFormattable formattable ? formattable.ToString(format, formatProvider) : arg?.ToString() ?? string.Empty;

    }

​

    public static class StringExtensions

    {

        public static string FormatString(byte[] buffer) => string.Create(new ExampleCustomFormatter(), stackalloc char[64], $"{buffer:S}");

​

        // How to make this work? Maybe needs to have TryWrite 

        // public static string FormatString2(ReadOnlySpan<byte> buffer) => string.Create(new ExampleCustomFormatter(), stackalloc char[64], $"{buffer:S}");

    }

​

    [InterpolatedStringHandler]

    public ref struct BinaryMessageInterpolatedStringHandler

    {

        private readonly DefaultInterpolatedStringHandler handler;

​

        public BinaryMessageInterpolatedStringHandler(int literalLength, int formattedCount, bool predicate, out bool handlerIsValid)

        {

            handler = default;

​

            if(predicate)

            {

                handlerIsValid = false;

                return;

            }

​

            handlerIsValid = true;

            handler = new DefaultInterpolatedStringHandler(literalLength, formattedCount);

        }

​

        public void AppendLiteral(string s) => handler.AppendLiteral(s);

​

        public void AppendFormatted<T>(T t) => handler.AppendFormatted(t);

​

        public override string ToString() => handler.ToStringAndClear();

    }

​

​

    public static void Main()

    {

        byte[] test1 = new byte[1] { 0x55 };

        ReadOnlySpan<byte> test2 = new byte[1] { 0x55 };

​

        // How to make this work? Now it prints "System.Byte[]".

        Console.WriteLine($"{test1:S}");

​

        Console.WriteLine(StringExtensions.FormatString(test1));

​

        // How to make this work? This does not compile.

        // Console.WriteLine($"{test2:2}");

​

        // How to make this work? This does not compile.

        // Console.WriteLine(StringExtensions.FormatString(test2));

    }

}