Serialize NaN values into JSON as nulls in JSON.NET

Question

Most Json parsers don't serialize NaN, because in Javascript, NaN is not a constant. Json.Net, however, does serialize NaN values into NaN, which means it outputs invalid Json; attempting to deserialize this Json will fail with most parsers. (We're deserializing in WebKit.)

We have hacked the Json.Net code to output null values when passed NaN, but this seems like a poor solution. Douglas Crockford (once) recommended using nulls in place of NaNs:

http://www.json.org/json.ppt (Look at slide 16)

Clearly this won't work in all cases, but it would be ok for our purposes. We'd just rather not have to modify the source code of Json.Net. Does anyone know how to use Json.Net to convert NaN inputs into null outputs?

Answer 1

The author advises us to “Write a JsonConverter for float/double to make NaN safe if this is important for you,” so that’s what you can do:

class LawAbidingFloatConverter : JsonConverter {
    public override bool CanRead
    {
        get
        {
            return false;
        }
    }
    public override bool CanWrite
    {
        get
        {
            return true;
        }
    }
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var val = value as double? ?? (double?) (value as float?);
        if (val == null || Double.IsNaN((double)val) || Double.IsInfinity((double)val))
        {
            writer.WriteNull();
            return;
        }
        writer.WriteValue((double)val);
    }
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(double) || objectType == typeof(float);
    }
}

and then use it:

var settings = new JsonSerializerSettings();
var floatConverter = new LawAbidingFloatConverter();
settings.Converters.Add(floatConverter);
var myConverter = new JsonNetSerializer(settings);

Answer 2

Raphael Schweikerts solution with float support:

public class StandardFloatConverter : JsonConverter
{
    public override bool CanRead
    {
        get
        {
            return false;
        }
    }
    public override bool CanWrite
    {
        get
        {
            return true;
        }
    }
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        if (value == null)
        {
            writer.WriteNull();
            return;
        }

        var val = Convert.ToDouble(value);
        if(Double.IsNaN(val) || Double.IsInfinity(val))
        {
            writer.WriteNull();
            return;
        }
        // Preserve the type, otherwise values such as 3.14f may suddenly be
        // printed as 3.1400001049041748.
        if (value is float)
            writer.WriteValue((float)value);
        else
            writer.WriteValue((double)value);
    }
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(double) || objectType == typeof(float);
    }
}

Answer 3

For the sake of future readers, if zeros are acceptable to you instead of nulls, it seems this issue has been addressed by Json.Net.

Serializing NaN and Infinity Floating Point Values

Json.NET no longer serializes NaN and positive and negative infinity floating point values as symbols, which is invalid JSON. With 5.0 the new default is to serialize those values as strings, e.g. "NaN" instead of NaN. There is no change to serializing normal floating point numbers.

A FloatFormatHandling setting has been added so you can control how NaN and infinity values are serialized.

string json;     
IList<double> d = new List<double> {1.1, double.NaN, double.PositiveInfinity};

json = JsonConvert.SerializeObject(d);

// [1.1,"NaN","Infinity"]

json = JsonConvert.SerializeObject(d, new JsonSerializerSettings {FloatFormatHandling = FloatFormatHandling.Symbol});  

// [1.1,NaN,Infinity]

json = JsonConvert.SerializeObject(d, new JsonSerializerSettings {FloatFormatHandling = FloatFormatHandling.DefaultValue});  

// [1.1,0.0,0.0]