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JSON and DateTime Kind

I've been working on a project for a client recently and was bitten by a bug recently.  It has to do with using the Microsoft AJAX Extensions, specifically, the JavascriptSerializer.  Our use of JSON is simply as an easy way to serialize our objects, whether it be to disk or across the wire.  What was happening is objects that contained dates would get their time shifted when they reached the other side.  The explanation is that the serialization format of JSON has no way of representing what .NET uses its Kind property for.  And since there is no Kind available on deserialization it assumes UTC. 

For those who don't know or don't memorize, here is a quick bit of trivia.  Assume central time.

  ToLocal ToUniversalTime
DateTime.Kind = Local Unchanged +6 hours
DateTime.Kind = Utc - 6 hours Unchanged
DateTime.Kind = Unspecified ? ?

The answer is...

 

 

 

 

A DateTime of Unspecified Kind that has ToLocal applied to it will be -6.  Similarly a DateTime of Unspecified Kind that has ToUniversalTime applied will be +6.  The logic is that .NET does not know the kind.  If you call ToLocal or ToUniversalTime you must want a conversion.  As to why the JSON deserialization process chooses to make it Utc I don't know.  It would seem to make more sense to simply use Unspecified as the json has not specified a kind.  There are several blog posts covering this subject (here and here for example) but the solutions offered seemed to come up short of what I desired.  I want to serialize any object and not have to inform the other developers that DateTime properties need this extra logic in their setters. 

Fortunately, the JavascriptSerializer allows you to add your own Converters with ease.  Information on how to use it can be found on Microsoft's site here.  However, when I first looked at it I was a bit confused by the SupportedTypes property, as I have never seen code quite like it.

        public override IEnumerable<Type> SupportedTypes
        {
            //Define the ListItemCollection as a supported type.
            get { return new ReadOnlyCollection<Type>(new List<Type>(new Type[] { typeof(ListItemCollection) })); }
        }

Can it really be necessary to create a new ReadOnlyCollection, passing it a new generic List, passing it a Type array, which specifies a Type?  Yeah, this is a bit more complicated than it needs to be!  I will post my entire console sample for you to play with yourself.  Hopefully it saves someone the trouble I went through.

 

using System;
using System.Collections.Generic;
using System.Web.Script.Serialization;

namespace jsonconverters
{
    class Program
    {
        static void Main(string[] args)
        {
            DateTime now = DateTime.SpecifyKind(DateTime.Now, DateTimeKind.Unspecified);
            
            Console.WriteLine(now.ToString());
            var title = new Title() { Name = "Iron Man", ReleaseDate = now, CreateDate = now };

            var serializer = new JavaScriptSerializer();
            
            //comment this line out to see results without our custom converter
            serializer.RegisterConverters(new JavaScriptConverter[] { new DateTimeConverter() });

            var json = serializer.Serialize(title);
            Console.WriteLine(json);
            var newTitle = serializer.Deserialize<Title>(json);
            Console.WriteLine(newTitle.Name);
            Console.WriteLine(newTitle.ReleaseDate);
            Console.WriteLine(newTitle.ReleaseDate == now);
            Console.WriteLine(newTitle.CreateDate);
            Console.WriteLine(newTitle.CreateDate == now);
            Console.WriteLine(newTitle.LastUpdate.HasValue);
            Console.ReadLine();

        }
    }

    public class Title
    {
        public string Name { get; set; }
        public DateTime ReleaseDate { get; set; }
        public DateTime? CreateDate { get; set; }
        public DateTime? LastUpdate { get; set; }
    }

    public class DateTimeConverter : JavaScriptConverter
    {
        private string _dateKey = "d";
        private string _kindKey = "k";
        public override IEnumerable<Type> SupportedTypes
        {
            get { return new List<Type>() { typeof(DateTime), typeof(DateTime?) }; }
        }

        public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)
        {
            Dictionary<string, object> result = new Dictionary<string, object>();
            if (obj.GetType() == typeof(DateTime))
            {
                DateTime d = (DateTime)obj;
                result[_dateKey] = d.Ticks;
                if (d.Kind != DateTimeKind.Unspecified)
                    result[_kindKey] = d.Kind;
                return result;

            }
            else if (obj.GetType() == typeof(DateTime?))
            {
                DateTime? theDate = (DateTime?)obj;
                if (theDate.HasValue)
                {
                    result[_dateKey] = theDate.Value.Ticks;
                    if (theDate.Value.Kind != DateTimeKind.Unspecified)
                        result[_kindKey] = theDate.Value.Kind;
                }
            }
            return result;
        }

        public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)
        {
            if (dictionary.ContainsKey(_dateKey))
            {
                var d = new DateTime(long.Parse(dictionary[_dateKey].ToString()), DateTimeKind.Unspecified);
                if (dictionary.ContainsKey(_kindKey))
                    d = DateTime.SpecifyKind(d, (DateTimeKind)dictionary[_kindKey]);
                return d;
            }
            return null;
        }
    }
}

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