.NET Core: points must form a closed linestring when turn coordinate to polygon

Question

I have list of geometry https://pastebin.ubuntu.com/p/zhgPJ4BkpK/ that I got from over pass turbo, Now I want to make polygon by .NET Core, in order to, after converting lat and long to coordinate Coordinate coordinate = new Coordinate(g.lon, g.lat);

I am trying to convert list to polygon:

    var coordinates = new List<Coordinate>();

    foreach (var geoElement in elements)
    {
        if (geoElement.type == "way")
        {
            foreach (var g in geoElement.geometry)
            {
                Coordinate coordinate = new Coordinate(g.lon, g.lat);
                coordinates.Add(coordinate);
            }
        }
        else
        {
            foreach (var m in geoElement.members)
            {
                if (m.type == "way")
                {
                    foreach (var g in m.geometry)
                    {
                        Coordinate coordinate = new Coordinate(g.lon, g.lat);
                        coordinates.Add(coordinate);
                    }
                }
            }
        }
    }

    var geometryFactory = new GeometryFactory(new PrecisionModel(), 4326);
    var polygon = geometryFactory.CreatePolygon(coordinates.ToArray());

    return polygon;
    

But I got this error:

Unhandled exception. System.ArgumentException: points must form a closed linestring

at NetTopologySuite.Geometries.LinearRing.ValidateConstruction()
at NetTopologySuite.Geometries.LinearRing..ctor(CoordinateSequence points, GeometryFactory factory)
at NetTopologySuite.Geometries.GeometryFactory.CreateLinearRing(CoordinateSequence coordinates)
at NetTopologySuite.Geometries.GeometryFactory.CreateLinearRing(Coordinate[] coordinates)
at NetTopologySuite.Geometries.GeometryFactory.CreatePolygon(Coordinate[] coordinates)

How can I fix this issue?

Answer 1

Add this line after the foreach, so it will create a closed linestring, which can be used to create a polygon.

coordinates.Add(coordinates[0]);