using System;

using System.IO;

using System.Threading;

using System.Threading.Tasks;

using Microsoft.Extensions.Hosting;

using Microsoft.Extensions.Configuration;

using Microsoft.Extensions.Logging;

using Microsoft.Extensions.DependencyInjection;

using Microsoft.Extensions.Options;

​

public class Program

{

    public class Settings

    {

        public string Test { get; set; }

    }

    public static async Task Main()

    {

        var builder = CreateHostBuilder();

        await builder.Build().RunAsync();

    }

    private static IHostBuilder CreateHostBuilder()

    {

        File.WriteAllText("appsettings.json", "{\"Test\": \"1\"}");

        // One host for all services

        return new HostBuilder()

            .UseEnvironment("BETA")

            .ConfigureAppConfiguration(builder => builder.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true))

            .ConfigureLogging(builder => builder.AddConsole())

            .ConfigureServices((ctx, services) =>

                                   services.AddOptions()

                                           .Configure<Settings>(ctx.Configuration)

                                           .AddHostedService<TestHost>());

    }

    public class TestHost : IHostedService 

    {

        private readonly IOptionsMonitor<Settings> _options;

        private readonly ILogger<TestHost> _logger;

        public TestHost(IOptionsMonitor<Settings> options, ILogger<TestHost> logger)

        {

            _options = options; 

            _logger = logger;   

        }

        public async Task StartAsync(CancellationToken token)

        {

            var i = 0;

            while(!token.IsCancellationRequested)

            {

                i++;

                _logger.LogInformation(_options.CurrentValue.Test);

                await Task.Delay(TimeSpan.FromSeconds(1), token);

                if(i == 2)

                {

                    _logger.LogWarning("Change 'Test' = 2");

                    File.WriteAllText("appsettings.json", "{\"Test\": \"2\"}");

                }

                if(i > 5)

                {

                    break;

                }

            }

        }

        public Task StopAsync(CancellationToken token)

        {

            return Task.CompletedTask;

        }

    }

}