Custom JavaScriptConverter for DateTime?

JavaScriptSerializer can definitely do what you desire.

It's possible to customize the serialization performed by JavaScriptSerializer for any type by creating a custom converter and registering it with the serializer. If you have a class called Person, we could create a converter like so:

public class Person{    public string Name { get; set; }    public DateTime Birthday { get; set; }}public class PersonConverter : JavaScriptConverter{    private const string _dateFormat = "MM/dd/yyyy";    public override IEnumerable<Type> SupportedTypes    {        get        {            return new[] { typeof(Person) };        }    }    public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)    {        Person p = new Person();        foreach (string key in dictionary.Keys)        {            switch (key)            {                case "Name":                    p.Name = (string)dictionary[key];                    break;                case "Birthday":                    p.Birthday = DateTime.ParseExact(dictionary[key] as string, _dateFormat, DateTimeFormatInfo.InvariantInfo);                    break;            }        }        return p;    }    public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)    {        Person p = (Person)obj;        IDictionary<string, object> serialized = new Dictionary<string, object>();        serialized["Name"] = p.Name;        serialized["Birthday"] = p.Birthday.ToString(_dateFormat);        return serialized;    }}

And use it like this:

JavaScriptSerializer serializer = new JavaScriptSerializer();serializer.RegisterConverters(new[] { new PersonConverter() });Person p = new Person            {                Name = "User Name",                Birthday = DateTime.Now            };string json = serializer.Serialize(p);Console.WriteLine(json);// {"Name":"User Name","Birthday":"12/20/2010"}Person fromJson = serializer.Deserialize<Person>(json);Console.WriteLine(String.Format("{0}, {1}", fromJson.Name, fromJson.Birthday)); // User Name, 12/20/2010 12:00:00 AM

Here's an enhancement for the accepted answer.

Using generics, passing a type and using reflection to determine the datetime properties.

public class ExtendedJavaScriptConverter<T> : JavaScriptConverter where T : new(){    private const string _dateFormat = "dd/MM/yyyy";    public override IEnumerable<Type> SupportedTypes    {        get        {            return new[] { typeof(T) };        }    }    public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)    {        T p = new T();        var props = typeof(T).GetProperties();        foreach (string key in dictionary.Keys)        {            var prop = props.Where(t => t.Name == key).FirstOrDefault();            if (prop != null)            {                if (prop.PropertyType == typeof(DateTime))                {                    prop.SetValue(p, DateTime.ParseExact(dictionary[key] as string, _dateFormat, DateTimeFormatInfo.InvariantInfo), null);                }                else                {                    prop.SetValue(p, dictionary[key], null);                }            }        }                          return p;    }          public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)    {        T p = (T)obj;        IDictionary<string, object> serialized = new Dictionary<string, object>();        foreach (PropertyInfo pi in typeof(T).GetProperties())        {            if (pi.PropertyType == typeof(DateTime))            {                serialized[pi.Name] = ((DateTime)pi.GetValue(p, null)).ToString(_dateFormat);            }            else            {                serialized[pi.Name] = pi.GetValue(p, null);            }        }        return serialized;    }    public static JavaScriptSerializer GetSerializer()     {        JavaScriptSerializer serializer = new JavaScriptSerializer();        serializer.RegisterConverters(new[] { new ExtendedJavaScriptConverter<T>() });        return serializer;    }}

Usage is simple:

 JavaScriptSerializer serialiser = ExtendedJavaScriptConverter<Task>.GetSerializer();

Hope that helps someone.

There is actually a nice clean way to do this without knowing the wrapper type or even needing a wrapper object.

You use JavaScriptConverter to convert your object to a Uri that also implements IDictionary. JavaScriptSerializer will serialize this as a string.

This hack is described here: Custom DateTime JSON Format for .NET JavaScriptSerializer