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

                return ms.ToArray();
            }
        }
    }
}