asp.net core配置文件加载过程的深入了解

前言

配置文件中程序运行中，担当着不可或缺的角色；通常情况下，使用 visual studio 进行创建项目过程中，项目配置文件会自动生成在项目根目录下，如 appsettings.json，或者是被大家广泛使用的 appsettings.{env.EnvironmentName}.json；配置文件

作为一个入口，可以让我们在不更新代码的情况，对程序进行干预和调整，那么对其加载过程的全面了解就显得非常必要。

何时加载了默认的配置文件

在 Program.cs 文件中，查看以下代码

 public class Program
 {
  public static void Main(string[] args)
  {
   CreateWebHostBuilder(args).Build().Run();
  }

  public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
   WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>();
 }

WebHost.CreateDefaultBuilder 位于程序集 Microsoft.AspNetCore.dll 内，当程序执行 WebHost.CreateDefaultBuilder(args) 的时候，在 CreateDefaultBuilder 方法内部加载了默认的配置文件

代码如下

public static IWebHostBuilder CreateDefaultBuilder(string[] args)
  {
   var builder = new WebHostBuilder();

   if (string.IsNullOrEmpty(builder.GetSetting(WebHostDefaults.ContentRootKey)))
   {
    builder.UseContentRoot(Directory.GetCurrentDirectory());
   }
   if (args != null)
   {
    builder.UseConfiguration(new ConfigurationBuilder().AddCommandLine(args).Build());
   }

   builder.UseKestrel((builderContext, options) =>
    {
     options.Configure(builderContext.Configuration.GetSection("Kestrel"));
    })
    .ConfigureAppConfiguration((hostingContext, config) =>
    {
     var env = hostingContext.HostingEnvironment;

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

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

     config.AddEnvironmentVariables();

     if (args != null)
     {
      config.AddCommandLine(args);
     }
    })
    .ConfigureLogging((hostingContext, logging) =>
    {
     logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
     logging.AddConsole();
     logging.AddDebug();
     logging.AddEventSourceLogger();
    })
    .ConfigureServices((hostingContext, services) =>
    {
     // Fallback
     services.PostConfigure<HostFilteringOptions>(options =>
     {
      if (options.AllowedHosts == null || options.AllowedHosts.Count == 0)
      {
       // "AllowedHosts": "localhost;127.0.0.1;[::1]"
       var hosts = hostingContext.Configuration["AllowedHosts"]?.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries);
       // Fall back to "*" to disable.
       options.AllowedHosts = (hosts?.Length > 0 ? hosts : new[] { "*" });
      }
     });
     // Change notification
     services.AddSingleton<IOptionsChangeTokenSource<HostFilteringOptions>>(
      new ConfigurationChangeTokenSource<HostFilteringOptions>(hostingContext.Configuration));

     services.AddTransient<IStartupFilter, HostFilteringStartupFilter>();
    })
    .UseIIS()
    .UseIISIntegration()
    .UseDefaultServiceProvider((context, options) =>
    {
     options.ValidateScopes = context.HostingEnvironment.IsDevelopment();
    });

   return builder;
  }								 
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