web-app-template/backend/app/app.Configuration/HostExtensions.cs

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.EventLog;
using System;
using System.IO;
using System.Reflection;
using System.Runtime.InteropServices;

namespace app.Configuration {
    public static class HostExtensions {
        // copied from https://github.com/dotnet/extensions/blob/release/3.1/src/Hosting/Hosting/src/Host.cs, 
        // modified to use customer environment
        public static IHostBuilder CreateBuilder(string[] args) {
            var builder = new HostBuilder();

            builder.UseContentRoot(Directory.GetCurrentDirectory()); // UseWindowsService overrides content root to AppContext.BaseDirectory
            builder.ConfigureHostConfiguration(config => {
                config.AddEnvironmentVariables(prefix: "DOTNET_");
                config.AddEnvironmentVariables(prefix: "LOGINCOUNTER_");
                if (args != null) {
                    config.AddCommandLine(args);
                }
            });

            builder.ConfigureAppConfiguration((hostingContext, config) => {
                var env = hostingContext.HostingEnvironment;
                var customerEnv = hostingContext.Configuration.GetValue<string>(Constants.CustomerEnvironmentVariable);
                //var customerEnv = Environment.GetEnvironmentVariable(Constants.CustomerEnvironmentVariable);
                if (!string.IsNullOrEmpty(customerEnv))
                    config.SetBasePath(Path.Combine(hostingContext.HostingEnvironment.ContentRootPath, Constants.CustomersDirectoryName, customerEnv));

                config.AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);
                config.AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: false, reloadOnChange: true);

                var computerName = Environment.GetEnvironmentVariable(Constants.ComputerNameEnvironmentVariable);
                if (!string.IsNullOrEmpty(computerName))
                    config.AddJsonFile($"appsettings.{computerName}.json", optional: true, reloadOnChange: true);

                if (env.IsDevelopment() && !string.IsNullOrEmpty(env.ApplicationName)) {
                    var appAssembly = Assembly.Load(new AssemblyName(env.ApplicationName));
                    if (appAssembly != null) {
                        config.AddUserSecrets(appAssembly, optional: true);
                    }
                }

                config.AddEnvironmentVariables();
                config.AddEnvironmentVariables("IOTHUB_");

                if (args != null) {
                    config.AddCommandLine(args);
                }
            })
            .ConfigureLogging((hostingContext, logging) => {
                var isWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);

                // IMPORTANT: This needs to be added *before* configuration is loaded, this lets
                // the defaults be overridden by the configuration.
                if (isWindows) {
                    // Default the EventLogLoggerProvider to warning or above
                    logging.AddFilter<EventLogLoggerProvider>(level => level >= LogLevel.Warning);
                }

                logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
                logging.AddConsole();
                logging.AddDebug();
                logging.AddEventSourceLogger();

                if (isWindows) {
                    // Add the EventLogLoggerProvider on windows machines
                    logging.AddEventLog();
                }
            })
            .UseDefaultServiceProvider((context, options) => {
                var isDevelopment = context.HostingEnvironment.IsDevelopment();
                options.ValidateScopes = isDevelopment;
                options.ValidateOnBuild = isDevelopment;
            });

            return builder;
        }

        public static void LogHostingEnvironment<T>(this IHost host) {
            var loggerFactory = host.Services.GetRequiredService<ILoggerFactory>();
            var configuration = host.Services.GetRequiredService<IConfiguration>();
            var hostEnvironment = host.Services.GetRequiredService<IHostEnvironment>();
            var logger = loggerFactory.CreateLogger<T>();
            logger.LogInformation("Running Host...");
            logger.LogInformation($"EnvironmentName:{hostEnvironment.EnvironmentName}");
            logger.LogInformation($"ContentRootPath:{hostEnvironment.ContentRootPath}");
            logger.LogInformation($"CurrentDirectory:{Environment.CurrentDirectory}");
            logger.LogInformation($"ApplicationName:{hostEnvironment.ApplicationName}");
            logger.LogInformation($"Customer:{configuration.GetCustomer()}");
            logger.LogInformation($"ComputerName:{configuration.GetComputerName()}");
        }

        public static void LogStartError(Exception ex) {
            var logDir = Path.Combine(AppContext.BaseDirectory, "logs");
            if (!Directory.Exists(logDir))
                Directory.CreateDirectory(logDir);
            var logFile = Path.Combine(logDir, "start_error.txt");
            File.AppendAllText(logFile, DateTime.Now.ToString() + ": " + ex.ToString() + Environment.NewLine);
        }

        public static void ChangeCurrentDirToProjectDir() {
            var currentDir = Directory.GetCurrentDirectory();
            var ix = currentDir.IndexOf("\\bin"); // goto project dir, if we are running from VS
            if (ix >= 0) {
                var newDir = currentDir.Substring(0, ix);
                Directory.SetCurrentDirectory(newDir);
            }
        }
    }
}