Visual

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

WebHostBuilder 知道该如何配置 Web Server 的环境，CreateDefaultBuilder 即进行默认配置。

默认配置

反编译的 CreateDefaultBuilder 方法源码：

/// <summary>
/// Initializes a new instance of the <see cref="T:Microsoft.AspNetCore.Hosting.WebHostBuilder" /> class with pre-configured defaults.
/// </summary>
public static IWebHostBuilder CreateDefaultBuilder(string[] args)
{
    WebHostBuilder hostBuilder = new WebHostBuilder();
    if (string.IsNullOrEmpty(hostBuilder.GetSetting(WebHostDefaults.ContentRootKey)))
    hostBuilder.UseContentRoot(Directory.GetCurrentDirectory());
    if (args != null)
    hostBuilder.UseConfiguration((IConfiguration) new ConfigurationBuilder().AddCommandLine(args).Build());
    hostBuilder.UseKestrel((Action<WebHostBuilderContext, KestrelServerOptions>) ((builderContext, options) => options.Configure((IConfiguration) builderContext.Configuration.GetSection("Kestrel")))).ConfigureAppConfiguration((Action<WebHostBuilderContext, IConfigurationBuilder>) ((hostingContext, config) =>
    {
    IHostingEnvironment hostingEnvironment = hostingContext.HostingEnvironment;
    config.AddJsonFile("appsettings.json", true, true).AddJsonFile("appsettings." + hostingEnvironment.EnvironmentName + ".json", true, true);
    if (hostingEnvironment.IsDevelopment())
    {
        Assembly assembly = Assembly.Load(new AssemblyName(hostingEnvironment.ApplicationName));
        if (assembly != (Assembly) null)
        config.AddUserSecrets(assembly, true);
    }
    config.AddEnvironmentVariables();
    if (args == null)
        return;
    config.AddCommandLine(args);
    })).ConfigureLogging((Action<WebHostBuilderContext, ILoggingBuilder>) ((hostingContext, logging) =>
    {
    logging.AddConfiguration((IConfiguration) hostingContext.Configuration.GetSection("Logging"));
    logging.AddConsole();
    logging.AddDebug();
    logging.AddEventSourceLogger();
    })).ConfigureServices((Action<WebHostBuilderContext, IServiceCollection>) ((hostingContext, services) =>
    {
    services.PostConfigure<HostFilteringOptions>((Action<HostFilteringOptions>) (options =>
    {
        if (options.AllowedHosts != null && options.AllowedHosts.Count != 0)
        return;
        string str = hostingContext.Configuration["AllowedHosts"];
        string[] strArray1;
        if (str == null)
        {
            strArray1 = (string[]) null;
        }
        else
        {
            char[] separator = new char[1]{ ';' };
            int num = 1;
            strArray1 = str.Split(separator, (StringSplitOptions) num);
        }
        string[] strArray2 = strArray1;
        HostFilteringOptions filteringOptions = options;
        string[] strArray3;
        if (strArray2 == null || strArray2.Length == 0)
        strArray3 = new string[1]{ "*" };
        else
        strArray3 = strArray2;
        filteringOptions.AllowedHosts = (IList<string>) strArray3;
    }));
    services.AddSingleton<IOptionsChangeTokenSource<HostFilteringOptions>>((IOptionsChangeTokenSource<HostFilteringOptions>) new ConfigurationChangeTokenSource<HostFilteringOptions>(hostingContext.Configuration));
    services.AddTransient<IStartupFilter, HostFilteringStartupFilter>();
    })).UseIIS().UseIISIntegration().UseDefaultServiceProvider((Action<WebHostBuilderContext, ServiceProviderOptions>) ((context, options) => options.ValidateScopes = context.HostingEnvironment.IsDevelopment()));
    return (IWebHostBuilder) hostBuilder;
}

• 使用 Kestrel Web Server
  • ASP.NET Core 内置，跨平台
• IIS 集成
  • UseIIS(), UseIISIntegration()
• Log
• IConfiguration 接口

IConfiguration 配置信息的来源

上面源码第一句 WebHostBuilder hostBuilder = new WebHostBuilder(); 创建 WebHostBuilder 对象时，就初始化了 Configuration（先配置 _config 字段，然后将值传入新建的 WebHostBuilderContext）：

/// <summary>
/// Initializes a new instance of the <see cref="T:Microsoft.AspNetCore.Hosting.WebHostBuilder" /> class.
/// </summary>
public WebHostBuilder()
{
    this._hostingEnvironment = new HostingEnvironment();
    this._configureServicesDelegates = new List<Action<WebHostBuilderContext, IServiceCollection>>();
    this._configureAppConfigurationBuilderDelegates = new List<Action<WebHostBuilderContext, IConfigurationBuilder>>();
    this._config = (IConfiguration) new ConfigurationBuilder().AddEnvironmentVariables("ASPNETCORE_").Build();
    if (string.IsNullOrEmpty(this.GetSetting(WebHostDefaults.EnvironmentKey)))
    this.UseSetting(WebHostDefaults.EnvironmentKey, Environment.GetEnvironmentVariable("Hosting:Environment") ?? Environment.GetEnvironmentVariable("ASPNET_ENV"));
    if (string.IsNullOrEmpty(this.GetSetting(WebHostDefaults.ServerUrlsKey)))
    this.UseSetting(WebHostDefaults.ServerUrlsKey, Environment.GetEnvironmentVariable("ASPNETCORE_SERVER.URLS"));
    this._context = new WebHostBuilderContext()
    {
    Configuration = this._config
    };
}

配置信息来源：

• appsettings.json
• User Secrets
• 系统环境变量
• 命令行参数

config.AddJsonFile("appsettings.json", true, true)
      .AddJsonFile("appsettings." + hostingEnvironment.EnvironmentName + ".json", true, true);
if (hostingEnvironment.IsDevelopment())
{
    Assembly assembly = Assembly.Load(new AssemblyName(hostingEnvironment.ApplicationName));
    if (assembly != (Assembly) null)
    config.AddUserSecrets(assembly, true);
}
config.AddEnvironmentVariables();
if (args == null)
    return;
config.AddCommandLine(args);

如果出现重复的属性，后添加的会覆盖前面的值，所以系统环境变量的优先级高于 appsettings.json。