Serializing FeatureCollection GEOJSON in .NET Core

Question

I am trying to serialize a FeatureCollection so I can pass it as a request body. To do this I am using a NetTopologySuite.IO.GeoJSON package and using the instruction they provide on their github page. The problem is, when hitting the jsonSerializer.Serialize method, I get an error saying: {"At least one element in the source array could not be cast down to the destination array type."} System.Exception {System.InvalidCastException}.

My method looks like this:

    private async Task<IEnumerable<VehicleArea>> GetVehiclesInAreas(FeatureCollection areas)
            {
                var uri = $"www.someurl/getByArea";
    
                using var httpClient = new HttpClient();
    
                string stringifiedAreas;
                var jsonSerializer = GeoJsonSerializer.CreateDefault();
                using (var stringWriter = new StringWriter())
                using (var jsonwriter = new Newtonsoft.Json.JsonTextWriter(stringWriter))
                {
                    jsonSerializer.Serialize(jsonwriter, areas);
                    stringifiedAreas = stringWriter.ToString();
                }
    
                StringContent content = new StringContent(stringifiedAreas, Encoding.UTF8, "application/json");
    
                var response = await httpClient.PostAsync(uri, content);
             }

The GEOSJON object I am passing in the request body of my controller, that ends up being the areas FeatureCollection provided to the given method looks like this:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "id": 16,  
      "geometry": {
        "type": "GeometryCollection",
        "geometries": [
          {
            "type": "Polygon",
            "coordinates": [
              [
[ 
                  12.1234,
                  55.1234
                ],
                [ 
                  12.7,
                  55.7],
                [ 
                  12.8,
                  55.8
                ],
                [ 
                  12.9,
                  55.9
                ],
                [ 
                  12.1234,
                  55.1234
                ]
              ]
            ]
          }
        ]
      }
    },
  {
      "type": "Feature",
      "id": 16,  
      "geometry": {
        "type": "GeometryCollection",
        "geometries": [
          {
            "type": "Polygon",
            "coordinates": [
              [
                [ 
                  12.1234,
                  55.1234
                ],
                [ 
                  12.7,
                  55.7],
                [ 
                  12.8,
                  55.8
                ],
                [ 
                  12.9,
                  55.9
                ],
                [ 
                  12.1234,
                  55.1234
                ]
              ]
            ]
          }
        ]
      }
    }
  ]
}

Finally, here is a snippet of how the areas object looks like during runtime:

You are passing a `FeatureCollection` containing `StjFeature`s. `StjFeature`s implement `IFeature` but belong to the `NetTopologySuite.IO.GeoJSON4STJ` package. That package uses `System.Text.Json`. Can you reimplement your code using functionality from `NetTopologySuite.IO.GeoJSON4STJ` and `System.Text.Json`? — FObermaier, Sep 01 '21 at 15:23

score 2 · Answer 1 · answered Sep 02 '21 at 10:54

Thanks to @FObermaier's comment, I was able to implement a solution. As he mentioned, I am using a NetTopologySuite.IO.GeoJSON4STJ package which uses the System.Text.Json. By refactoring my code to use System.Text.Json.JsonSerializer, I was able to make it work. For anyone encountering the same issue, make sure to also pass the options object with the GeoJsonConverter factory added as a converter. Here is the code example:

    var options = new JsonSerializerOptions();

    options.Converters.Add(new NetTopologySuite.IO.Converters.GeoJsonConverterFactory());

    var serialized = JsonSerializer.Serialize(areas, options);
    var stringifiedAreas = serialized.ToString();

Serializing FeatureCollection GEOJSON in .NET Core

1 Answers1