Deserialize Open Street Map JSON in C#

Question

If I have this JSON where there is a header with the tags version, generator, om3s and elements. elements can be of type node or way and the associated JSON keys vary based on type. I am trying to use JsonSubTypes to convert each element type to a C# class.

Example JSON:

[
  {
    "version": 0.6,
    "generator": "Overpass API 0.7.55.7 8b86ff77",
    "osm3s": {
      "timestamp_osm_base": "2019-05-21T18:03:02Z",
      "copyright": "The data included in this document is from www.openstreetmap.org. The data is made available under ODbL."
    },
    "elements": [
      {
        "type": "node",
        "id": 4949106384,
        "lat": 32.2686857,
        "lon": -107.738218,
        "tags": {
          "highway": "turning_circle"
        }
      },
      {
        "type": "way",
        "id": 14527404,
        "nodes": [
          142882281,
          3048075541,
          1598998260
        ],
        "tags": {
          "highway": "residential",
          "name": "West Apple Street",
          "tiger:cfcc": "A41",
          "tiger:county": "Luna, NM",
          "tiger:name_base": "Apple",
          "tiger:name_direction_prefix": "W",
          "tiger:name_type": "St",
          "tiger:reviewed": "no"
        }
      }
    ]
  }
]

I am trying to deserialize it using:

var json = JsonConvert.DeserializeObject<OSMdata>(jsonText);

Where OSMdata looks like:

[JsonConverter(typeof(JsonSubtypes), "type")]
[JsonSubtypes.KnownSubType(typeof(Element.Node), "node")]
[JsonSubtypes.KnownSubType(typeof(Element.Edge), "way")]

public abstract class OSMdata
{
    public float version { get; set; }
    public string generator { get; set; }
    public Osm3s osm3s { get; set; }
    public Element[] elements { get; set; }
}

public class Osm3s : OSMdata
{
    public DateTime timestamp_osm_base { get; set; }
    public string copyright { get; set; }
}

public class Element : OSMdata
{
    public class Node : Element
    {
        public string type { get; } = "node";
        public long id { get; set; }
        public float lat { get; set; }
        public float lon { get; set; }
        public NodeTags tags { get; set; }
    }

    public class NodeTags : Node
    {
        public string highway { get; set; }
        public string _ref { get; set; }
    }

    public class Edge : Element
    {
        public string type { get; } = "way";
        public long id { get; set; }
        public long[] nodes { get; set; }
        public EdgeTags tags { get; set; }
    }

    public class EdgeTags : Edge
    {
        public string highway { get; set; }
        public string name { get; set; }
        public string cfcc { get; set; }
        public string county { get; set; }
        public string oneway { get; set; }
    }
}

Which returns:

Unhandled Exception: System.ArgumentNullException: Value cannot be null.
   at System.RuntimeType.MakeGenericType(Type[] instantiation)
   at JsonSubTypes.JsonSubtypes.CreateCompatibleList(Type targetContainerType, Type elementType)
   at JsonSubTypes.JsonSubtypes.ReadArray(JsonReader reader, Type targetType, JsonSerializer serializer)
   at JsonSubTypes.JsonSubtypes.ReadJson(JsonReader reader, Type objectType, JsonSerializer serializer)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)
   at newapp.Program.Main(String[] args) in C:\Users\RDCRLDDH\source\repos\newapp\newapp\Program.cs:line 23

While I don't understand this error and am looking for a solution, here are a few questions I would like clarity on:

Questions

Am I properly constructing the class OSMdata? I think I am following the examples correctly, but am not sure if I am properly assigning the classes Node and Edge to the parent class OSMdata.

How will the deserializer know to assign the tag "tiger:cfcc" to the Cfcc property in EdgeTags?

An alternative solution is to add a `NodeOrEdge` class and write custom cast between them. Let the library handles the missing keys. — SGKoishi, May 23 '19 at 23:29
Rather than loading to some huge `dynamic` object then manually deserializing, you can deserialize to model with a polymorphic `element` array. For how see [Json.Net Serialization of Type with Polymorphic Child Object](https://stackoverflow.com/q/29528648/3744182), [How to implement custom JsonConverter in JSON.NET to deserialize a List of base class objects?](https://stackoverflow.com/q/8030538/3744182) or [Deserializing polymorphic json classes without type information using json.net](https://stackoverflow.com/q/19307752/3744182). — dbc, May 23 '19 at 23:40
Anytime you use `dynamic` always think to your self the is most likely a better way — TheGeneral, May 24 '19 at 00:44
@dubbbdan - you're welcome. Did those answer your question? If not you can always use `JToken.ToObject()` or `JToken.ToObject()` as shown in [Deserializing JToken content to an Object](https://stackoverflow.com/q/28492098/344280/) and [Converting a JToken (or string) to a given Type](https://stackoverflow.com/a/20407670/344280). Hopefully your question gets answered by one of these other questions. — dbc, May 24 '19 at 00:53

score 1 · Answer 1 · answered May 29 '19 at 23:39

Declare your classes as follows:


    // no longer abstract
    public class OSMdata
    {
        public float version { get; set; }
        public string generator { get; set; }
        public Osm3s osm3s { get; set; }

        // for arrays or collection this line must be present here
        [JsonConverter(typeof(JsonSubtypes), "type")]
        public Element[] elements { get; set; }
    }

    // no need to inherits from OSMData
    public class Osm3s
    {
        public DateTime timestamp_osm_base { get; set; }
        public string copyright { get; set; }
    }


    [JsonConverter(typeof(JsonSubtypes), "type")]
    [JsonSubtypes.KnownSubType(typeof(Node), "node")]
    [JsonSubtypes.KnownSubType(typeof(Edge), "way")]
    public abstract class Element : OSMdata
    {
        public abstract string type { get; }
    }

    public class Node : Element
    {
        public override string type { get; } = "node";
        public long id { get; set; }
        public float lat { get; set; }
        public float lon { get; set; }
        public NodeTags tags { get; set; }
    }

    public class NodeTags
    {
        public string highway { get; set; }
        public string _ref { get; set; }
    }

    public class Edge : Element
    {
        public override string type { get; } = "way";
        public long id { get; set; }
        public long[] nodes { get; set; }
        public EdgeTags tags { get; set; }
    }

    public class EdgeTags
    {
        public string highway { get; set; }
        public string name { get; set; }
        public string cfcc { get; set; }
        public string county { get; set; }
        public string oneway { get; set; }
    }

and deserialize with:

var json = JsonConvert.DeserializeObject<OSMdata>(jsonText);

see running sample: https://dotnetfiddle.net/pdJ0ab

Would it be appropriate to associate `EdgeTags` with `Edge` (i.e. `public class EdgeTags : Edge`)? — dubbbdan, May 30 '19 at 13:43

score 0 · Accepted Answer · answered May 24 '19 at 20:41

I couldn't figure out how to deserialize directly from the JSON, but found a suitable workaround that converts the JSON to JArray and iterates though each element and converting to a C# class that is composed of nodes and edges.

Using the class Element:

    [JsonConverter(typeof(JsonSubtypes), "type")]
    [JsonSubtypes.KnownSubType(typeof(Element.Node), "node")]
    [JsonSubtypes.KnownSubType(typeof(Element.Edge), "way")]

    public class Element
    {
        public class Node : Element

        {
            public string type { get; } = "node";
            public long id { get; set; }
            public float lat { get; set; }
            public float lon { get; set; }
            public NodeTags tags { get; set; }
        }

    public class NodeTags : Node
    {
        public string highway { get; set; }
        public string _ref { get; set; }
    }

    public class Edge : Element
    {
        public string type { get; } = "way";
        public long id { get; set; }
        public long[] nodes { get; set; }
        public EdgeTags tags { get; set; }
    }

    public class EdgeTags : Edge
    {
        [JsonProperty("highway")]
        public string Highway { get; set; }

        [JsonProperty("name")]
        public string Name { get; set; }


        [JsonProperty("tiger:cfcc")]
        public string cfcc { get; set; }

        [JsonProperty("tiger:county")]
        public string County { get; set; }

        [JsonProperty("oneway")]
        public string Oneway { get; set; }
    }

You can parse thje json using:

JArray jsonSearch = JArray.Parse(jsonText);

You can then create a list consisting of each element using:

IList<JToken> results = jsonSearch[0]["elements"].Children().ToList();

Then you can iterate through results and convert the data to C# objects using:

var element_list = new List<Element>();

foreach (JObject element in results)
{
    Element myelement = element.ToObject<Element>();
    element_list.Add(myelement);
}

Of the questions I posed, the second is still relevant.

You can assign a property with an invalid name to a C# class using JsonProperty before you create the property name within the class.

[JsonProperty("tiger:cfcc")]
public string cfcc { get; set; }

Thanks @dbc and other helpful comments for pointing me in the right direction!

Deserialize Open Street Map JSON in C#

Questions

2 Answers2