Example.cs

using System;
using System.Collections.Generic;
using System.Text.Json; // Default .NET Core 3 JSON library
using BAMCIS.GeoJSON; // GeoJSON library

namespace Example
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = "[{\"id\": 2, \"address\": \"whatever\", \"longtitude\": 1.131, \"latitude\": 2.12}, {\"id\": 3, \"address\": \"whatever\", \"longtitude\": 0.321, \"latitude\": -1.321}]";
            string output = ApiResponseToGeoJSON(input);
            Console.WriteLine(output);
        }

        public static string ApiResponseToGeoJSON(string apiResponse)
        {
            // Deserialize JSON response to a POCO (Plain Old C# Object)
            ApiPosition[] apiPositions = JsonSerializer.Deserialize<ApiPosition[]>(apiResponse);

            // Create a list of Feature (types Feature, Point, Position and FeatureCollection are provided by BAMICS.GeoJSON)
            List<Feature> mapFeatures = new List<Feature>();

            // For each position from the response create a Feature and add it to previously created list
            foreach (ApiPosition apiPosition in apiPositions)
            {
                mapFeatures.Add(new Feature(
                    new Point(new Position(apiPosition.latitude, apiPosition.longtitude)),
                    new Dictionary<string, dynamic>() { { "address", apiPosition.address } }
                ));
            }

            // Use the list to build a FeatureCollection
            FeatureCollection mapFeatureCollection = new FeatureCollection(mapFeatures);

            // FeatureCollection instances provides the method ToJson which converts them to Json strings, use it before sending the FeatureCollection to the frontend
            return mapFeatureCollection.ToJson();
        }
    }

    // This type is used to serialize input JSON, it should match input JSON members 
    public class ApiPosition
    {
        public int id { get; set; }
        public string address { get; set; }
        public double longtitude { get; set; }
        public double latitude { get; set; }
    }
}