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

[Shaaruddin]

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]
        ]]]
    }
}
  ]
}