Question

我尝试创建自定义DateTime转换器失败。问题：我有很多要序列化的对象，其中包含DateTime的一些包含DateTime.MinValue的属性。我想将它序列化为null。但是我要求在对象内部装饰正确的所有解决方案（我不能这样做）我在下面找到的其他解决方案是创建转换器，据我所知，这个转换器只能显式返回DateTime对象，而不能在其他对象中返回。请帮忙。

public class DateTimeConverter : JsonConverter
{
    private readonly Type[] types;

    public DateTimeConverter(params Type[] types)
    {
        this.types = types;
    }

    public override bool CanConvert(Type objectType)
    {
        return types.Any(t => t == objectType);
    }
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        JToken t = JToken.FromObject(value);

        if (t.Type != JTokenType.Object)
        {
            if (value is DateTime && value.Equals(DateTime.MinValue))
            {
                t = JToken.FromObject(null);
                t.WriteTo(writer);
            }
            else
            {
                t.WriteTo(writer);
            }
        }
        else
        {
            if (value.Equals(DateTime.MinValue)) {
                t = JToken.FromObject(null);
                t.WriteTo(writer);
            }
            else {
                t.WriteTo(writer);
            }
        }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException("Unnecessary because CanRead is false. The type will skip the converter.");
    }

Answer 1

我认为你过分复杂了。您的转换器应该只担心转换日期值，而不是转换可能包含日期值的对象。只要在任何其他对象内遇到日期，序列化器就会自然地调用转换器。

这是一个简单的转换器，可以执行您想要的操作：

public class MinDateToNullConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        // This converter handles date values directly
        return (objectType == typeof(DateTime));
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        // The CanConvert method guarantees the value will be a DateTime
        DateTime date = (DateTime)value;
        if (date == DateTime.MinValue)
        {
            writer.WriteNull();
        }
        else
        {
            writer.WriteValue(date);
        }
    }

    public override bool CanRead
    {
        get { return false; }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

这是一个演示，显示转换器在整个嵌套的对象层次结构中的日期工作：

class Program
{
    static void Main(string[] args)
    {
        Example example = new Example
        {
            Date1 = new DateTime(2014, 2, 2),
            Date2 = DateTime.MinValue,
            Inner = new Inner
            {
                DateA = DateTime.MinValue,
                DateB = new DateTime(1954, 1, 26)
            },
            MoreDates = new List<DateTime>
            {
                new DateTime(1971, 11, 15),
                DateTime.MinValue
            }
        };

        // Set up the serializer to use our date converter
        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.Converters.Add(new MinDateToNullConverter());
        settings.Formatting = Formatting.Indented;

        string json = JsonConvert.SerializeObject(example, settings);
        Console.WriteLine(json);
    }
}

class Example
{
    public DateTime Date1 { get; set; }
    public DateTime Date2 { get; set; }
    public Inner Inner { get; set; }
    public List<DateTime> MoreDates { get; set; }
}

class Inner
{
    public DateTime DateA { get; set; }
    public DateTime DateB { get; set; }
}

输出：

{
  "Date1": "2014-02-02T00:00:00",
  "Date2": null,
  "Inner": {
    "DateA": null,
    "DateB": "1954-01-26T00:00:00"
  },
  "MoreDates": [
    "1971-11-15T00:00:00",
    null
  ]
}

Answer 2

当转换器实现CanConvert（和你的）时，你可以将其添加到序列化设置：

JsonConvert.SerializeObject(foo, new JsonSerializerSettings {
    Converters = {
        new DateTimeConverter()
    }
});

这样，它将应用于它支持的所有对象。

Answer 3

找到我的问题的答案。实际上我在JsonSerializer设置中有2个转换器。我注意到当我删除包含DateTime MinValue属性的对象A上的转换器时，我的序列化器工作正常。这让我明白DateTime转换为null不能在其他转换器下工作。所以我所做的是手动转换对象A转换器，而不是完全干净的解决方案，我希望DateTimeToNull转换器也可以在A转换器内工作。这是我在A转换器中所做的：

    public class AConverter : JsonConverter
{

    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(A));
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        JToken jToken = JToken.FromObject(value);
        JObject jObject = (JObject)jToken;
        A aInfo = (A) value;
        string statusString = aInfo.Status == null ? string.Empty : SomeUtil.GetStateString((State)aInfo.Status.State, aInfo.Status.Downloading);
        jObject.AddFirst(new JProperty("MachineState", statusString));

        if (aInfo.UploadTime == DateTime.MinValue) {
            jObject.Remove("UploadTime");
            jObject.Add(new JProperty("UploadTime", null));
        }
        jObject.WriteTo(writer);
    }

    public override bool CanRead
    {
        get
        {
            return false;
        }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException("It's OK!");
    }


}

使用Json.Net自定义DateTime序列化

3 个答案: