EDITORIAL
The ASP .NET Core generic host builder is introduced in v2.1, the application builder starts when the host, the host as a target for packaging applications and resources, and starts application lifecycle management. Its main functions include initializing configuration (including loading configuration and configured to convert the format of the common key), and create a managed environment Host general context, dependency injection.
Adopted in .NET Core 3.0 IHostBuilder used to create Host, but also no longer recommend the use of Web hosting and recommend a generic host, mainly due to the original generic host apply only to non-HTTP load, in order to provide more extensive hosting plan, it is necessary to separate the pipeline and HTTP Web interface to the host. But still backward compatible Web host.
.NET Core 3.0 to create a generic host
The following code is the code template provided in V3.0, you can see the process of creating host, it has abandoned the way of creation of WebHostBuilder
1: public class Program
2: {
3: public static void Main(string[] args)
4: {
5: CreateHostBuilder(args).Build().Run();
6: }
7:
8: public static IHostBuilder CreateHostBuilder(string[] args) =>
9: Host.CreateDefaultBuilder(args)
10: .ConfigureWebHostDefaults(webBuilder =>
11: {
12: webBuilder.UseStartup<Startup>();
13: });
14: }
In the .NET Core 2.X
1: public class Program
2: {
3: public static void Main(string[] args)
4: {
5: CreateWebHostBuilder(args).Build().Run();
6: }
7:
8: public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
9: WebHost.CreateDefaultBuilder(args)
10: .UseStartup<Startup>();
11: }
V3.0模板中提供的CreateHostBuilder()方法看起来非常类似于V2.X中的CreateWebHostBuilder()。
其主要区别在于对WebHost.CreateDefaultBuilder()由Host.CreateDefaultBuilder()替换。使用CreateDefaultBuilder()辅助方法可以非常轻松地从v2.x切换到v3.0。
另一个区别是关于ConfigureWebHostDefaults()的调用。由于新的主机构建器是通用主机构建器，因此我们必须让它知道我们打算为Web主机配置默认设置。这些默认配置我们可以在ConfigureWebHostDefaults()方法中实现
CreateDefaultBuilder
该方法Microsoft.Extensions.Hosting.Host中，它是一个静态类，里面有两个方法，一个有参的CreateDefaultBuilder(string[] args)，一个是无参的。
无参方法源码如下，
1: public static IHostBuilder CreateDefaultBuilder() =>
2: CreateDefaultBuilder(args: null);
可以看到该方法实际上是设置了默认值。
IHostBuilder CreateDefaultBuilder(string[] args)方法主要有以下功能：
创建HostBuilder对象
1: var builder = new HostBuilder();
指定Host要使用的内容根目录
1: builder.UseContentRoot(Directory.GetCurrentDirectory());
配置初始化(环境变量、appsettings.json、User Secrets)
1: builder.ConfigureHostConfiguration(config =>
2: {
3: config.AddEnvironmentVariables(prefix: "DOTNET_");
4: if (args != null)
5: {
6: config.AddCommandLine(args);
7: }
8: });
9:
10: builder.ConfigureAppConfiguration((hostingContext, config) =>
11: {
12: var env = hostingContext.HostingEnvironment;
13:
14: config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
15: .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true);
16:
17: if (env.IsDevelopment() && !string.IsNullOrEmpty(env.ApplicationName))
18: {
19: var appAssembly = Assembly.Load(new AssemblyName(env.ApplicationName));
20: if (appAssembly != null)
21: {
22: config.AddUserSecrets(appAssembly, optional: true);
23: }
24: }
25:
26: config.AddEnvironmentVariables();
27:
28: if (args != null)
29: {
30: config.AddCommandLine(args);
31: }
32: })
日志
1: .ConfigureLogging((hostingContext, logging) =>
2: {
3: logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
4: logging.AddConsole();
5: logging.AddDebug();
6: logging.AddEventSourceLogger();
7: })
在开发环境模式下启用作用域验证
1: .UseDefaultServiceProvider((context, options) =>
2: {
3: var isDevelopment = context.HostingEnvironment.IsDevelopment();
4: options.ValidateScopes = isDevelopment;
5: options.ValidateOnBuild = isDevelopment;
6: });
Build
Build()方法是Microsoft.Extensions.Hosting中，并且该方法只会执行一次，当然这种一次只是在同一个实例里面
1: public IHost Build()
2: {
3: if (_hostBuilt)
4: {
5: throw new InvalidOperationException("Build can only be called once.");
6: }
7: _hostBuilt = true;
8:
9: BuildHostConfiguration();
10: CreateHostingEnvironment();
11: CreateHostBuilderContext();
12: BuildAppConfiguration();
13: CreateServiceProvider();
14:
15: return _appServices.GetRequiredService<IHost>();
16: }
该方法主要是包括以下功能：
创建HostingEnvironment
创建HostBuilderContext
配置初始化及格式标准化
DI(创建IHostEnvironment、IHostApplicationLifetime、IHostLifetime、IHost)
Run
Run方法运行应用程序并阻止调用线程，直到主机关闭
1: public static void Run(this IHost host)
2: {
3: host.RunAsync().GetAwaiter().GetResult();
4: }
以下是RunAsync的源码，此处可以通过设置CancellationToken的值，使应用程序自动关闭
1: public static async Task RunAsync(this IHost host, CancellationToken token = default)
2: {
3: try
4: {
5: await host.StartAsync(token);
6:
7: await host.WaitForShutdownAsync(token);
8: }
9: finally
10: {
11: #if DISPOSE_ASYNC
12: if (host is IAsyncDisposable asyncDisposable)
13: {
14: await asyncDisposable.DisposeAsync();
15: }
16: else
17: #endif
18: {
19: host.Dispose();
20: }
21:
22: }
23: }