JSON.Net JsonConverter for DbGeography

Question

Long long struggles with this...

Basically I have a model-first EF5 object with a DbGeography property. I would like to apply a JsonConverter that let's it roundtrip as simple latitude/longitude values. I'm using WebAPI.

Looking for JSON output and input like so:

{
    "location":
    {
        "geopoint":
        {
            "latitude":40.770712,
            "longitude":-73.962011
        }
    }
}

Here is my class definition and JsonConverter:

[MetadataType(typeof(QueryLocationMetadata))]
partial class Location
{
}

public class QueryLocationMetadata
{
    [JsonConverter(typeof(DbGeographyConverter))]
    public virtual DbGeography GeoPoint { get; set; }
}


public class DbGeographyConverter : JsonConverter
{
    private const string LATITUDE_KEY = "latitude";
    private const string LONGITUDE_KEY = "longitude";

    public override bool CanConvert(Type objectType)
    {
        return objectType.Equals(typeof(DbGeography));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return default(DbGeography);

        var jObject = JObject.Load(reader);

        if (!jObject.HasValues || (jObject.Property(LATITUDE_KEY) == null || jObject.Property(LONGITUDE_KEY) == null))
            return default(DbGeography);

        string wkt = string.Format("POINT({1} {0})", jObject[LATITUDE_KEY], jObject[LONGITUDE_KEY]);
        return DbGeography.FromText(wkt, DbGeography.DefaultCoordinateSystemId);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var dbGeography = value as DbGeography;

        serializer.Serialize(writer, dbGeography == null || dbGeography.IsEmpty ? null : new { latitude = dbGeography.Latitude.Value, longitude = dbGeography.Longitude.Value });
    }
}

So using this I'm able to successfully serialize and even deserialize an object correctly but before I ever hit my ApiController action I get the following error:

System.Data.SqlTypes.SqlNullValueException: Data is Null. This method or property cannot be called on Null values.
   at System.Data.SqlTypes.SqlDouble.get_Value()
   at GetValueFromSqlDouble(Object )
   at System.Web.Http.Metadata.Providers.AssociatedMetadataProvider`1.<>c__DisplayClass3.<GetMetadataForPropertiesImpl>b__0()
   at System.Web.Http.Metadata.ModelMetadata.get_Model()
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateProperties(ModelMetadata metadata, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateProperties(ModelMetadata metadata, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateProperties(ModelMetadata metadata, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateProperties(ModelMetadata metadata, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateElements(IEnumerable model, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateProperties(ModelMetadata metadata, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateElements(IEnumerable model, ValidationContext validationContext)
   at System.Web.Http.Validation.DefaultBodyModelValidator.ValidateNodeAndChildren(ModelMetadata metadata, ValidationContext validationContext, Object container)
   at System.Web.Http.Validation.DefaultBodyModelValidator.Validate(Object model, Type type, ModelMetadataProvider metadataProvider, HttpActionContext actionContext, String keyPrefix)
   at System.Web.Http.ModelBinding.FormatterParameterBinding.<>c__DisplayClass1.<ExecuteBindingAsync>b__0(Object model)
   at System.Threading.Tasks.TaskHelpersExtensions.<>c__DisplayClass36`1.<>c__DisplayClass38.<Then>b__35()
   at System.Threading.Tasks.TaskHelpersExtensions.<>c__DisplayClass49.<ToAsyncVoidTask>b__48()
   at System.Threading.Tasks.TaskHelpers.RunSynchronously[TResult](Func`1 func, CancellationToken cancellationToken)

After toying around and googling all sorts of things, I am absolutely at a loss. I get that in general it is attempting to validate the property but a SqlDouble?

Sim

Answer 1

The issue isn't with your converter. This is a known validation bug that happens when a public property's getter throws in the object graph. This work item tracks the issue:

http://aspnetwebstack.codeplex.com/workitem/740

In the mean time, you should be able to get around it by disabling validation:

config.Services.Clear(typeof(ModelValidatorProvider));

Sorry for the inconvenience.

Demi

Temporary or not, at least that lets me keep some of the hair on my head. Thanks! – Sim

You may have to use the fully qualified name: System.Web.Http.Validation.ModelValidatorProvider as mentioned here: aspnetwebstack.codeplex.com/workitem/55 – Heartbreak

@Youssef, please have a look at the link in my answer. I'm curious to know if this is a known issue. – Dentition

Really ? my hate toward MSSQL is getting over 9000 – Merciless

Answer 2

The bug mentioned by Youssef has since been fixed, but I still ran into issues when creating a custom JsonConverter for DbGeography in that somewhere in the validation process the DefaultBodyModelValidator got stuck in a loop. My solution was not to disable model validation altogether, but to replace the default validator with a derived one that excludes the DbGeography type from deep validation.

So as not to repeat myself, you can see the full solution here.

Dentition

you saved me another day of rage, this DbGeogrphy is .... really the worthless type i ever seen in my life. – Merciless

8 years later we still run into new problems that it causes across our systems. To some extent we regret leveraging it. – Dentition