How to add altitude coordinates in Feature Collection then serialize to Geojson

Question

I greatly appreciate for any help, I deserialized GeoJson file into feature collection using GeoJson.Net. Then I get the feature from the feature collection. Inside the feature I can loop and get all the type of geometry. From there I can loop each of the type and get the coordinate. However now I am stuck on how to edit to add Altitude equal to 0 for each coordinate then serialize back to geojson.

This is my sample test.geojson file:

{
  "type": "FeatureCollection",
  "name": "MYS_adm2",
  "crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },
  "features": [
{
    "type": "Feature",
    "id": 0,    
    "properties": {"party": "Republican","count": "2500"},
    "geometry": {
        "type": "Polygon",
        "coordinates": [[
            [-104.05, 48.99],
            [-97.22,  48.98],
            [-96.58,  45.94],
            [-104.03, 45.94],
            [-104.05, 48.99]
        ]]
    }
},
{
    "type": "Feature",
    "id": 1,
    "properties": {"party": "Democrat","count": "1300"},
    "geometry": {
        "type": "MultiPolygon",
        "coordinates": [[[
            [-109.05, 41.00],
            [-102.06, 40.99],
            [-102.03, 36.99],
            [-109.04, 36.99],
            [-109.05, 41.00]
        ]]]
    }
}
  ]
}

This is code how I deserialized:

string filePath = @"D:\test.geojson";
            FeatureCollection collection = JsonConvert.DeserializeObject<FeatureCollection>(File.ReadAllText(filePath)); //1 min 21 s
            var feature = collection.Features;
            foreach (var featureItem in feature)
            {
                if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.Point))
                {
                    Point point = featureItem.Geometry as Point;

                }
                else if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.MultiPoint))
                {

                }
                else if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.Polygon))
                {
                    Polygon polygon = featureItem.Geometry as Polygon;
                    foreach (var Item in polygon.Coordinates)
                    {
                        foreach (var coordinates in Item.Coordinates)
                        {
                            //Here I want to add value 0 altitude coordinate
                            

                        }
                    }
                }
                else if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.MultiPolygon))
                {
                    MultiPolygon multiPolygon = featureItem.Geometry as MultiPolygon;
                    foreach (var Item in multiPolygon.Coordinates)
                    {
                        foreach (var item1 in Item.Coordinates)
                        {
                            foreach (var item2 in item1.Coordinates)
                            {
                                //Here I want to add value 0 altitude coordinate
                            }
                        }
                    }
                }
            }

            //here I want to serialize my FeatureCollection after edit
            File.WriteAllText(@"D:\test_Edit.geojson", JsonConvert.SerializeObject(collection));

My expected output is to add Altitude equal to 0 like below:

{
  "type": "FeatureCollection",
  "name": "MYS_adm2",
  "crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },
  "features": [
{
    "type": "Feature",
    "id": 0,    
    "properties": {"party": "Republican","count": "2500"},
    "geometry": {
        "type": "Polygon",
        "coordinates": [[
            [-104.05, 48.99, 0],
            [-97.22,  48.98, 0],
            [-96.58,  45.94, 0],
            [-104.03, 45.94, 0],
            [-104.05, 48.99, 0]
        ]]
    }
},
{
    "type": "Feature",
    "id": 1,
    "properties": {"party": "Democrat","count": "1300"},
    "geometry": {
        "type": "MultiPolygon",
        "coordinates": [[[
            [-109.05, 41.00, 0],
            [-102.06, 40.99, 0],
            [-102.03, 36.99, 0],
            [-109.04, 36.99, 0],
            [-109.05, 41.00, 0]
        ]]]
    }
}
  ]
}

Answer 1

Types and some properties are closed from modification, most likely to preserve data integrity. Therefore, to change the values ​​of some properties, they need to be recreated. Having changed your version, I got the following code:

static void Main(string[] args)
        {
            string filePath = @"C:\Users\aleks\Documents\test.geojson.txt";
            FeatureCollection collection = JsonConvert.DeserializeObject<FeatureCollection>(File.ReadAllText(filePath)); //1 min 21 s
            var feature = collection.Features;

            var editedFeatures = new List<Feature>();
            IGeometryObject editedObject = null;

            foreach (var featureItem in feature)
            {
                if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.Point))
                {
                    Point point = featureItem.Geometry as Point;
                }
                else if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.MultiPoint))
                {
                }
                else if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.Polygon))
                {
                    var polygon = featureItem.Geometry as Polygon;

                    var editedLines = new List<LineString>();
                    foreach (var Item in polygon.Coordinates)
                    {
                        var editedcoordinates = new List<Position>();
                        foreach (var coordinates in Item.Coordinates)
                        {
                            editedcoordinates.Add(new Position(coordinates.Latitude, coordinates.Longitude, 0));
                        }
                        editedLines.Add(new LineString(editedcoordinates));
                    }

                    editedObject = new Polygon(editedLines);
                }
                else if (featureItem.Geometry.Type.Equals(GeoJSONObjectType.MultiPolygon))
                {
                    MultiPolygon multiPolygon = featureItem.Geometry as MultiPolygon;

                    var editedPoligons = new List<Polygon>();
                    foreach (var Item in multiPolygon.Coordinates)
                    {
                        var editedLines = new List<LineString>();
                        foreach (var item1 in Item.Coordinates)
                        {
                            var editedcoordinates = new List<Position>();
                            foreach (var item2 in item1.Coordinates)
                            {
                                editedcoordinates.Add(new Position(item2.Latitude, item2.Longitude, 0));
                            }
                            editedLines.Add(new LineString(editedcoordinates));
                        }
                        editedPoligons.Add(new Polygon(editedLines));
                    }
                    editedObject = new MultiPolygon(editedPoligons);
                }

                if (editedObject != null)
                {
                    editedFeatures.Add(new Feature(editedObject, featureItem.Properties, featureItem.Id));
                }
            }

            var editedCollection = new FeatureCollection(editedFeatures);
            editedCollection.CRS = collection.CRS;
            editedCollection.BoundingBoxes = collection.BoundingBoxes;
            //here I want to serialize my FeatureCollection after edit
            File.WriteAllText(@"C:\Users\aleks\Documents\test_Edit.geojson", JsonConvert.SerializeObject(editedCollection));
        }