SystemExtensions/SystemExtensions.NetStandard/Extensions/BytesExtensions.cs

using System.IO;

namespace System.Extensions
{
    public static class BytesExtensions
    {
        public static byte[] ReadAllBytes(this Stream stream)
        {
            if (stream is null)
            {
                throw new ArgumentNullException(nameof(stream));
            }

            var buffer = new byte[256];
            using var ms = new MemoryStream();
            int read;
            while ((read = stream.Read(buffer, 0, 256)) > 0)
            {
                ms.Write(buffer, 0, read);
            }

            return ms.ToArray();
        }
    }
}