C# Stream Extensions for reading/writing protocol buffers

ProtoBufStreamExtensions.cs

using System;
using System.IO;
using Google.Protobuf;

namespace ProtoCore
{
    public static class StreamExtensions
    {
        public static void Write(this Stream stream, IMessage message)
        {
            byte[] messageData = message.ToByteArray();
            stream.WriteLength(messageData.Length);
            stream.Write(messageData, 0, messageData.Length);
        }

        public static T Read<T>(this Stream stream) where T : IMessage<T>, new()
        {
            int length = stream.ReadLength();
            var messageData = new byte[length];
            stream.Read(messageData, 0, length);
            var parser = new MessageParser<T>(() => new T());
            return parser.ParseFrom(messageData, 0, length);
        }

        private static void WriteLength(this Stream stream, int length)
        {
            if (length < 64)
                stream.WriteByte((byte)length);
            else if (length < 16384)
            {
                var encodedLength = (ushort)(((length & 0xFF) << 8) | (length >> 8) | 0x40);
                stream.Write(BitConverter.GetBytes(encodedLength), 0, sizeof(ushort));
            }
            else if (length < 1_073_741_824)
            {
                long longLen = length;
                var encodedLength = (ulong) (
                    ((longLen & 0xFF) << 24) |
                    ((longLen & 0xFF00) << 8) |
                    ((longLen & 0xFF0000) >> 8) |
                    ((longLen & 0xFF000000) >> 24) | 0x80);
                stream.Write(BitConverter.GetBytes(encodedLength), 0, sizeof(ulong));
            }
            else
                throw new ArgumentException($"Length ({length}) is too long to be represented in byte stream.", nameof(length));
        }

        private static int ReadLength(this Stream stream)
        {
            int hiByte = stream.ReadByte();

            if ((hiByte & 0x80) != 0)
            {
                var lenData = new byte[4];
                lenData[3] = (byte) (hiByte & 0x3F);
                lenData[2] = (byte) stream.ReadByte();
                lenData[1] = (byte) stream.ReadByte();
                lenData[0] = (byte) stream.ReadByte();
                int len = BitConverter.ToInt32(lenData, 0);
                return len;
            }
            
            if ((hiByte & 0x40) != 0)
            {
                var lenData = new byte[2];
                lenData[1] = (byte) (hiByte & 0x3F);
                lenData[0] = (byte)stream.ReadByte();
                int len = BitConverter.ToInt16(lenData, 0);
                return len;
            }

            return hiByte;
        }
    }
}