HexEncoder.cs

public static class HexEncoder
{
    private static ReadOnlySpan<byte> HexTable => new[]
    {
        (byte)'0', (byte)'1', (byte)'2', (byte)'3',
        (byte)'4', (byte)'5', (byte)'6', (byte)'7',
        (byte)'8', (byte)'9', (byte)'A', (byte)'B',
        (byte)'C', (byte)'D', (byte)'E', (byte)'F'
    };

    [SkipLocalsInit]
    public static unsafe string Encode(ReadOnlySpan<byte> bytes)
    {
        var length = bytes.Length * 2;
        var span = length < 512 ? stackalloc char[length] : new char[length];
        ref var buffer = ref MemoryMarshal.GetReference(span);
        ref var hex = ref MemoryMarshal.GetReference(HexTable);

        for (var i = 0; i < bytes.Length; i++)
        {
            var b = bytes[i];
            buffer = (char)Unsafe.Add(ref hex, b >> 4);
            buffer = ref Unsafe.Add(ref buffer, 1);
            buffer = (char)Unsafe.Add(ref hex, b & 0xF);
            buffer = ref Unsafe.Add(ref buffer, 1);
        }

        return new string(span);
    }

    public static string Encode2(ReadOnlyMemory<byte> source) =>
        String.Create(source.Length * 2, source, static (span, src) =>
        {
            ref var buffer = ref MemoryMarshal.GetReference(span);
            ref var hex = ref MemoryMarshal.GetReference(HexTable);

            var bytes = src.Span;
            for (var i = 0; i < bytes.Length; i++)
            {
                var b = bytes[i];
                buffer = (char)Unsafe.Add(ref hex, b >> 4);
                buffer = ref Unsafe.Add(ref buffer, 1);
                buffer = (char)Unsafe.Add(ref hex, b & 0xF);
                buffer = ref Unsafe.Add(ref buffer, 1);
            }
        });
}