Grunter Grunter - 1 month ago 9
C# Question

Deserializing DbGeometry with Newtonsoft.Json

I'm building a SPA using Angular,Breeze and Web API 2 following the approach as outlined by John Papa in his latest PluralSight course.

Everything works well and I can pull information, update, insert, delete back to the server. However I'm using Spatial Types, and when I try to update an entity with a spatial type I get the following error


An exception of type 'Newtonsoft.Json.JsonSerializationException' occurred in
Newtonsoft.Json.dll but was not handled in user code

Additional information: Error getting value from 'WellKnownValue' on
'System.Data.Entity.Spatial.DbGeometry'.


The inner exception seems to point to the fact that the WellKnownValue is null, its not though, as I have checked the JSON being sent to the server which is then sent to the Breeze ContextProvider and saved using the SaveChanges method.

{
"entities": [
{
"TableKey": 2,
"CaseName": "Mikhail Lermontov",
"StartDate": "2013-06-11T00:00:00Z",
"EndDate": null,
"IsCurrent": true,
"SRID": 109,
"Shape": {
"$id": "2",
"$type": "System.Data.Entity.Spatial.DbGeometry, EntityFramework",
"Geometry": {
"$id": "3",
"$type": "System.Data.Entity.Spatial.DbGeometryWellKnownValue, EntityFramework",
"CoordinateSystemId": 2193,
"WellKnownText": "POLYGON ((1695943 5462665, 1713098 5462665, 1713098 5449659, 1695943 5449659, 1695943 5462665))"
}
},
"SpillLocation": "Marlborough Sounds",
"Image": "http://www.nzmaritime.co.nz/images/lm5.jpg\r\n",
"DefaultBaseMapKey": 2,
"__unmapped": {
"isPartial": false
},
"entityAspect": {
"entityTypeName": "DatSpillCase:#Osiris.Model",
"defaultResourceName": "DatSpillCases",
"entityState": "Modified",
"originalValuesMap": {
"CaseName": "Mikhail Lermontov"
},
"autoGeneratedKey": {
"propertyName": "TableKey",
"autoGeneratedKeyType": "Identity"
}
}
}
],
"saveOptions": {}
}


So my question is, is possible to deserialize DbGeometry types within the NewtonSoft library, and if not, what suggestions are there to get around that.

Answer

System.Data.Spatial.DbGeometry does not play nicely with Newtonsoft.Json

You need to create a JsonConverter to convert the DbGeometry

public class DbGeometryConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return objectType.IsAssignableFrom(typeof(string));
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            JObject location = JObject.Load(reader);
            JToken token = location["Geometry"]["WellKnownText"];
            string value = token.ToString();

            DbGeometry converted = DbGeometry.PolygonFromText(value, 2193);
            return converted;
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            // Base serialization is fine
            serializer.Serialize(writer, value);
        }
    }

Then on your property in your model add the attribute

[JsonConverter(typeof(DbGeometryConverter))]
public DbGeometry Shape { get; set; }

Now when you hit your BreezeController the deserialization will be handled by our new DbGeometryConverter.

Hope it helps.