NewtonSoft Json deserialize decimal numbers with more then 8 decimals - json.net

Question

I get the following json
"location": {
"x": 3693779.702309093,
"y": 500061.05750159378
}
And the following class
Class Location
public property x as string
public property y as string
end class
I need the json to be deserialised with all decimal positions
But when I deserialise with NewtonSoft Json, it is rounded to 8 decimals only.
Initially I had the class defined with x, y as decimals but I thought the rounding is caused by the data type, that's why I changed to string.
Still, it seems the actual Json deserialization is performed with only 8 decimals.
Any idea how can I make it to deserialize using all decimals?
Thank you

Answer 1

You may have found a bug in Json.NET itself. When reading a numeric value into a string, the value is first parsed to a numeric type (double, long or BigInteger), then back-converted to a string in JsonReader.ReadAsStringInternal():
string s;
if (Value is IFormattable)
s = ((IFormattable)Value).ToString(null, Culture);
else
s = Value.ToString();
SetToken(JsonToken.String, s, false);
return s;
The problem seems to be that Double.ToString(String, IFormatProvider) does not use round-trip precision unless the format string is explicitly specified to be "G17". If instead I use
System.ComponentModel.TypeDescriptor.GetConverter(Value).ConvertToString(null, Culture, Value)
there is no precision loss.
You might want to report an issue to NewtonSoft about this.
There are a couple of workarounds for this:
Deserialize your class with JsonSerializerSettings.FloatParseHandling = FloatParseHandling.Decimal. In this case the numeric value will be temporarily parsed to a decimal rather than a double.
Write a converter that loads the value into a JValue, checks for the value being of type JTokenType.Float, and returns the correct string:
public class StringNumericConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(string);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var value = JToken.Load(reader);
if (value.Type == JTokenType.Float)
return System.ComponentModel.TypeDescriptor.GetConverter(((JValue)value).Value).ConvertToInvariantString(((JValue)value).Value);
return (string)value;
}
public override bool CanWrite { get { return false; } }
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Or in VB.NET:
Public Class StringNumericConverter
Inherits JsonConverter
Public Overrides Function CanConvert(objectType As Type) As Boolean
Return objectType = GetType(String)
End Function
Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object
If reader.TokenType = JsonToken.Null Then
Return Nothing
End If
Dim value = JToken.Load(reader)
If value.Type = JTokenType.Float Then
Return System.ComponentModel.TypeDescriptor.GetConverter(DirectCast(value, JValue).Value).ConvertToInvariantString(DirectCast(value, JValue).Value)
End If
Return CType(value, String)
End Function
Public Overrides ReadOnly Property CanWrite() As Boolean
Get
Return False
End Get
End Property
Public Overrides Sub WriteJson(writer As JsonWriter, value As Object, serializer As JsonSerializer)
Throw New NotImplementedException()
End Sub
End Class
Note that for integral strings JValue.Value will be an Int64 or a BigInteger; you can't assume it will be a double.
Prototype fiddle.

Answer 2

I figured out the solution myself.
First I created the convertor below
public Class JsonGeometryConverter
Inherits JsonConverter
Public Overrides Function CanConvert(objectType As Type) As Boolean
Return True
End Function
Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object
Dim valF As double = reader.Value
Dim valFS = String.Format("{0:G17}", valF)
Return valFS
End Function
Public Overrides Sub WriteJson(writer As JsonWriter, value As Object, serializer As JsonSerializer)
Throw New NotImplementedException()
End Sub
End Class
Then I apply the converter attribute to the properties to extract values to
Public Class RestGIS_LocationInfo
<JsonConverter(GetType(JsonGeometryConverter))>
Public Property X As string = ""
<JsonConverter(GetType(JsonGeometryConverter))>
Public Property Y As String = ""
End Class
The converter reads the actual value from json data as double, and returns it formatted with up to 17 digits of precision (see https://msdn.microsoft.com/en-us/library/kfsatb94.aspx for details)