在ASP.NET Core中使用托管启动（hosting startup）程序集，实现批量注册service

在启动ASPNET Core时可以从外部程序集向应用添加增强功能。例如，外部库可以用托管启动（ hosting startup） 实现为应用程序提供附加配置（Configuration）或服务（service）。

具体实现如下：

1、实现 IHostingStartup 接口

2、标注程序集（HostingStartup）属性。

[assembly: HostingStartup(typeof(StartupEnhancement.StartupEnhancementHostingStartup))]

3、在CreateHostBuilder中配置加载的程序集，如果多个程序集 分号 隔开

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    //增加外部启动项Fap.Core.DI.ServicesInjection，初始化所有service
                    webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey,"Fap.Core")
                    
                    .UseStartup<Startup>();
                });

如果阻止Hosting Startup加载，需要以下设置

 webBuilder.UseSetting(WebHostDefaults.PreventHostingStartupKey, "true")

如果排除某些程序集的Hosting Startup加载

 webBuilder.UseSetting(WebHostDefaults.HostingStartupExcludeAssembliesKey, "ASSEMBLY1;ASSEMBLY2; ...")

多个程序集 分号 隔开。

ASPNET Core 中的 DI 没有批量注册service的功能。下面我就实现一个批量注册service的功能。

采用注解的形式，在需要注册为service的类上进行标注。

定义一个Attribute

    [AttributeUsage(AttributeTargets.Class)]
    public class ServiceAttribute:Attribute
    {
        public ServiceAttribute(ServiceLifetime serviceLifetime)
        {
            ServiceLifetime = serviceLifetime;
        }
        public ServiceLifetime ServiceLifetime { get; set; } = ServiceLifetime.Transient;
    }

定义需要注册的类和接口

    public interface IUser
    {
        string Get(string userName);
    }
    public interface IUser1
    {
        string Get1(string userName);
    }

    [Service(Microsoft.Extensions.DependencyInjection.ServiceLifetime.Singleton)]
    public class User : IUser,IUser1
    {
        private string s = Guid.NewGuid().ToString();
        public string Get(string userName)
        {
            return $"{s}===={userName}";
        }

        public string Get1(string userName)
        {
            return s;
        }
    }

如上，在User类上标注ServiceAttribute属性，设置ServiceLifetime为单利模式ServiceLifetime.Singleton

接下来实现 利用host startup来实现自动注册功能。根据class上标注的service attribute 来自动注册service

//标注程序集属性 HostingStartup
[assembly: HostingStartup(typeof(Fap.Core.DI.ServicesInjection))]
namespace Fap.Core.DI
{
    public class ServicesInjection : IHostingStartup
    {
        public void Configure(IWebHostBuilder builder)
        {
            builder.ConfigureServices(services =>
            {
                var basePath = AppDomain.CurrentDomain.RelativeSearchPath ?? AppDomain.CurrentDomain.BaseDirectory;
                var assemblies = Directory.GetFiles(basePath, "*.dll").Select(Assembly.LoadFrom).ToArray();
                var types = assemblies.SelectMany(a => a.DefinedTypes).Select(type => type.AsType()).Where(t => t.GetCustomAttribute<ServiceAttribute>() != null).ToArray();
                var implementTypes = types.Where(t => t.IsClass).ToArray();
                foreach (var implementType in implementTypes)
                {
                    var interfaceTypes = implementType.GetInterfaces();
                    foreach (var interfaceType in interfaceTypes)
                    {
                        var attr = implementType.GetCustomAttribute<ServiceAttribute>();
　　　　　　　　　　　　　　//根据serviceLifetime来向容器中注册
                        _ = (attr.ServiceLifetime switch
                        {
                            ServiceLifetime sl when sl == ServiceLifetime.Scoped => services.AddScoped(interfaceType, implementType),
                            ServiceLifetime sl when sl == ServiceLifetime.Singleton => services.AddSingleton(interfaceType, implementType),
                            ServiceLifetime sl when sl == ServiceLifetime.Transient => services.AddTransient(interfaceType, implementType),
                            _ => throw new FileNotFoundException("未找到此类型")
                        });
                    }
                }
            });
        }
    }
}

下面在Host builder时设置HostStartup

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    //增加外部启动项Fap.Core.DI.ServicesInjection，初始化所有service
                    webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey,"Fap.Core")
                    
                    .UseStartup<Startup>();
                });

这样我们在Controller中就可以使用已经自动注册到servicecontainer中的service了。

   public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;
        private readonly IUser1 _userService1;
        private readonly IUser _userService;
        public HomeController(ILogger<HomeController> logger,IUser1 userService1,IUser user)
        {
            _logger = logger;
            _userService1 = userService1;
            _userService = user;
        }

        public IActionResult Index()
        {
            ViewBag.CC = _userService.Get("zhangsan")+_userService1.Get1("lisi");
            return View();
        }
}

除了利用IHostingStartup为应用提供服务注册，还可以提供额外配置。

[assembly: HostingStartup(typeof(HostingStartupLibrary.ServiceKeyInjection))]

namespace HostingStartupLibrary
{
    public class ServiceKeyInjection : IHostingStartup
    {
        public void Configure(IWebHostBuilder builder)
        {
            builder.ConfigureAppConfiguration(config =>
            {
                var dict = new Dictionary<string, string>
                {
                    {"DevAccount_FromLibrary", "DEV_1111111-1111"},
                    {"ProdAccount_FromLibrary", "PROD_2222222-2222"}
                };

                config.AddInMemoryCollection(dict);
            });
        }
    }
}

在controller中就可以访问到如上配置项

        public IndexModel(IConfiguration config)
        {
            ServiceKey_Development_Library = config["DevAccount_FromLibrary"];
            ServiceKey_Production_Library = config["ProdAccount_FromLibrary"];
          
        }

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具体源码实现：

webBuilder.UseSetting(WebHostDefaults.HostingStartupAssembliesKey,"Fap.Core")

设置了WebHostOptions中的HostingStartupAssemblies属性，存放我们要加载的IHostingStartup的程序集。

在IWebHostBuilder 调用 Builder()返回IWebHost方法中进行调用。下面为关键代码

            _options = new WebHostOptions(_config, Assembly.GetEntryAssembly()?.GetName().Name);
　　　　　　　//没有设置阻止加载webBuilder.UseSetting( WebHostDefaults.PreventHostingStartupKey, "false")
            if (!_options.PreventHostingStartup)
            {
                var exceptions = new List<Exception>();

                // 执行 hosting startup assemblies
                foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
                {
                    try
                    {
                        var assembly = Assembly.Load(new AssemblyName(assemblyName));

                        foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
                        {
　　　　　　　　　　　　　　　　　//实例化自定义的hostingStartup
                            var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType);
　　　　　　　　　　　　　　　　　//调用Configure方法，执行我们自定的逻辑
                            hostingStartup.Configure(this);
                        }
                    }
                    catch (Exception ex)
                    {
                        // Capture any errors that happen during startup
                        exceptions.Add(new InvalidOperationException($"Startup assembly {assemblyName} failed to execute. See the inner exception for more details.", ex));
                    }
                }

                if (exceptions.Count > 0)
                {
                    hostingStartupErrors = new AggregateException(exceptions);
                }
            }

_options.GetFinalHostingStartupAssemblies()方法代码如下：

   public IEnumerable<string> GetFinalHostingStartupAssemblies()
        {
　　　　　　　//返回HostingStartupAssemblies中排除掉HostingStartupExcludeAssemblies的程序集
            return HostingStartupAssemblies.Except(HostingStartupExcludeAssemblies, StringComparer.OrdinalIgnoreCase);
        }