使用JavaScriptSerializer进行JSON序列化时自定义JavaScriptConverter的问题

有史以来第一次发这么长的题目，先汗一个~~~
要序列化的类如下

public class TestAAA

{

    public string Name

    {

        get;

        set;

    }

    public DateTime? BirthDay

    {

        get;

        set;

    }  

}

TestAAA obj = new TestAAA

        {

            Name = "张三",

            BirthDay = DateTime.Now

        };

        JavaScriptSerializer jss = new JavaScriptSerializer();

        TextBox1.Text= jss.Serialize(obj);

在使用JavaScriptSerializer序列化后得到这么个字符串
{"Name":"张三","Birthday":"\/Date(1340693123252)\/"}
因为想要用yyyyMMdd的方式进行序列化输出，因此想到了自定义JavaScriptConverter

public class DateTimeConverter : JavaScriptConverter

{



    public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)

    {

        if (string.IsNullOrEmpty(dictionary["Value"].ToString()))

            return null;



        return DateTime.Parse(dictionary["Value"].ToString());

    }



    public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)

    {



        IDictionary<string, object> result = new Dictionary<string, object>();

        if (obj == null)

            result["Value"] = string.Empty;

        else

            result["Value"] = ((DateTime)obj).ToString("yyyy-MM-dd HH:mm:ss");

        return result;

    }



    public override IEnumerable<Type> SupportedTypes

    {

        get { yield return typeof(DateTime); }

    }

}

        JavaScriptSerializer jss = new JavaScriptSerializer();

        jss.RegisterConverters(new JavaScriptConverter[] { new DateTimeConverter() });

        TextBox1.Text= jss.Serialize(obj);

序列化后得到这样的结果
{"Name":"张三","BirthDay":{"Value":"2012-06-26 17:11:13"}}

可是反序列化的时候，问题出来了
JavaScriptSerializer jss = new JavaScriptSerializer();
jss.RegisterConverters(new JavaScriptConverter[] { new DateTimeConverter() });
TestAAA obj = jss.Deserialize<TestAAA>(TextBox1.Text);

在
TestAAA obj = jss.Deserialize<TestAAA>(TextBox1.Text);
这一句报了“未将对象引用设置到对象的实例”的错误

把反序列化代码改成这样，并加上断点
JavaScriptSerializer jss = new JavaScriptSerializer();
jss.RegisterConverters(new JavaScriptConverter[] { new DateTimeConverter() });
object obj = jss.DeserializeObject(TextBox1.Text);
TestAAA a = jss.ConvertToType<TestAAA>(obj);

第3行能正常执行，得到的obj是一个IDictionary<string,object>集合
第4行报“未将对象引用设置到对象的实例”的错误

错误的内部堆栈如下：
System.Web.Script.Serialization.ObjectConverter.ConvertDictionaryToObject(IDictionary`2 dictionary, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) +1059
System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeInternal(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) +71
System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) +147
System.Web.Script.Serialization.ObjectConverter.AssignToPropertyOrField(Object propertyValue, Object o, String memberName, JavaScriptSerializer serializer, Boolean throwOnError) +202
System.Web.Script.Serialization.ObjectConverter.ConvertDictionaryToObject(IDictionary`2 dictionary, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) +1177
System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeInternal(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) +71
System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject) +147
System.Web.Script.Serialization.ObjectConverter.ConvertObjectToType(Object o, Type type, JavaScriptSerializer serializer) +21
System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit) +48
System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(String input) +70


reflector反进去看了一下，没看出道道来，为啥这个反序列化不行？？？