using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var mc = new MyClass();

        mc.MyBinaryProperty = new byte[] { 1, 2, 3, 4 };

        string serialized = JsonConvert.SerializeObject(mc, new JsonSerializerSettings

        {

            TypeNameHandling = TypeNameHandling.Objects,

            Formatting = Newtonsoft.Json.Formatting.Indented,

            Converters = new[] { new ByteArrayConverter() }

        });

        Console.WriteLine (serialized);

    }

}

class MyClass

{

    public byte[] MyBinaryProperty {get;set;}

}

​

public class ByteArrayConverter : JsonConverter

{

    public override object ReadJson(

        JsonReader reader,

        Type objectType,

        object existingValue,

        JsonSerializer serializer)

    {

        throw new NotImplementedException();

    }

    public override void WriteJson(

        JsonWriter writer,

        object value,

        JsonSerializer serializer)

    {

        string base64String = Convert.ToBase64String((byte[])value);

        serializer.Serialize(writer, base64String);

    }    

    public override bool CanRead

    {

        get { return false; }

    }

    public override bool CanConvert(Type t)

    {

        return typeof(byte[]).IsAssignableFrom(t);

    }

}