针对DbGeography的JSON.Net JsonConverter

Question

漫长而漫长的斗争……

基本上，我有一个具有DbGeography属性的模型优先的EF5对象。我想应用一个JsonConverter，让它往返于简单的纬度/经度值。我使用的是WebAPI。

如下所示查找JSON输出和输入：

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

下面是我的类定义和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 });
    }
}

因此，使用它，我能够成功地序列化甚至反序列化一个对象，但是在我执行ApiController操作之前，我得到了以下错误：

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)

在玩弄和搜索了各种各样的东西之后，我完全不知所措。我知道通常它是在尝试验证属性，但是SqlDouble？

Answer 1

问题不在于您的转换器。这是一个已知的验证错误，当公共属性的getter在对象图中抛出时就会发生。此工作项跟踪问题：

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

同时，您应该能够通过禁用验证来绕过它：

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

很抱歉给您带来不便。

Answer 2

Youssef提到的错误已经修复了，但我在为DbGeography创建自定义JsonConverter时仍然遇到了问题，因为在验证过程中的某个地方，DefaultBodyModelValidator陷入了循环。我的解决方案不是完全禁用模型验证，而是用一个派生的验证器替换默认的验证器，该验证器将DbGeography类型排除在深度验证之外。

为了不重复我的话，you can see the full solution here.