Serialize Newtonsoft JSON to byte array

JSON is a character based format so there is necessarily character data involved. I suspect you used UTF16 encoding which makes each char into two bytes. If you use UTF8 you will not experience any meaningful size overhead.

I found a way to do what I wanted, its not precisely JSON, buts is BSON or also known as Binary JSON. Well since finding the solution was a pure luck, I am not sure how well known is BSON.

Anyway Newtonsoft supports it via Newtonsoft.Json.Bson nuget package at https://www.nuget.org/packages/Newtonsoft.Json.Bson/1.0.1

Some code for serialization/deserialization

foreach (var message in transportMessageList)
{
    MemoryStream ms = new MemoryStream();
    using (BsonDataWriter writer = new BsonDataWriter(ms))
    {
        JsonSerializer serializer = new JsonSerializer();
        serializer.Serialize(writer, message);
    }

    var bsonByteArray = ms.ToArray();
    Assert.True(bsonByteArray.Length!=0);
    bsonList.Add(bsonByteArray);
}

var deserializdTransmortMessageList = new List<TransportMessage>();
foreach (var byteArray in bsonList)
{
    TransportMessage message;
    MemoryStream ms = new MemoryStream(byteArray);
    using (BsonDataReader reader = new BsonDataReader(ms))
    {
        JsonSerializer serializer = new JsonSerializer();
        message = serializer.Deserialize<TransportMessage>(reader);
    }
    Assert.True(message.Transportdata.Length!=0);
    deserializdTransmortMessageList.Add(message);
}

You can use same classes/objects you use for JSON, serializing compressed array of bytes no longer cause an increase in size.

Please note that BSON documentation at Newtonsoft website is out dated and lists only deprecated api calls at this moment. My code uses proper non deprecated API calls.