//判断是否配置了StartupAssembly参数
if (!string.IsNullOrEmpty(webHostOptions.StartupAssembly))
{
  try
  {
    //根据你配置的程序集去查找Startup
    var startupType = StartupLoader.FindStartupType(webHostOptions.StartupAssembly, webhostContext.HostingEnvironment.EnvironmentName);
    UseStartup(startupType, context, services);
  }
  catch (Exception ex) when (webHostOptions.CaptureStartupErrors)
  {
    //此处省略代码省略
  }
}

StartupAssembly = configuration[WebHostDefaults.StartupAssemblyKey];

public static readonly string StartupAssemblyKey = "startupAssembly";

public static IHostBuilder CreateHostBuilder(string[] args) =>
      Host.CreateDefaultBuilder(args)
        .ConfigureHostConfiguration(config=> {
          List<KeyValuePair<string, string keyValuePairs = new List<KeyValuePair<string, string();
          //配置Startup所在的程序集名称
          keyValuePairs.Add(new KeyValuePair<string, string>("startupAssembly", "Startup所在的程序集名称"));
          config.AddInMemoryCollection(keyValuePairs);
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
          //这样的话这里就可以省略了
          //webBuilder.UseStartup<Startup>();
        });

public static Type FindStartupType(string startupAssemblyName, string environmentName)
{
  var assembly = Assembly.Load(new AssemblyName(startupAssemblyName));
  //名称Startup+环境变量的类比如(StartupDevelopment)
  var startupNameWithEnv = "Startup" + environmentName;
  //名称为Startup的类
  var startupNameWithoutEnv = "Startup";

  // 先查找包含名称Startup+环境变量的相关类，如果找不到则查找名称为Startup的类
  var type =
    assembly.GetType(startupNameWithEnv) "." + startupNameWithEnv) "." + startupNameWithoutEnv);

  if (type == null)
  {
    // 如果上述规则找不到，则在程序集定义的所有类中继续查找
    var definedTypes = assembly.DefinedTypes.ToList();

    var startupType1 = definedTypes.Where(info => info.Name.Equals(startupNameWithEnv, StringComparison.OrdinalIgnoreCase));
    var startupType2 = definedTypes.Where(info => info.Name.Equals(startupNameWithoutEnv, StringComparison.OrdinalIgnoreCase));

    var typeInfo = startupType1.Concat(startupType2).FirstOrDefault();
    if (typeInfo != null)
    {
      type = typeInfo.AsType();
    }
  }
  //最终返回Startup类型
  return type;
}

//创建Startup实例
object instance = ActivatorUtilities.CreateInstance(new HostServiceProvider(webHostBuilderContext), startupType);

private class HostServiceProvider : IServiceProvider
{
  private readonly WebHostBuilderContext _context;
  public HostServiceProvider(WebHostBuilderContext context)
  {
    _context = context;
  }

  public object GetService(Type serviceType)
  {
    // 通过这里我们就比较清晰的看出，只有满足这几种情况下才能返回具体的实例，其他的都会返回null
    #pragma warning disable CS0618 // Type or member is obsolete
    if (serviceType == typeof(Microsoft.Extensions.Hosting.IHostingEnvironment)
      || serviceType == typeof(Microsoft.AspNetCore.Hosting.IHostingEnvironment)
    #pragma warning restore CS0618 // Type or member is obsolete
      || serviceType == typeof(IWebHostEnvironment)
      || serviceType == typeof(IHostEnvironment)
      )
    {
      return _context.HostingEnvironment;
    }
    if (serviceType == typeof(IConfiguration))
    {
      return _context.Configuration;
    }
    //不满足这几种情况的类型都返回null
    return null;
  }
}

//传递startupType和环境变量参数查找返回ConfigureServicesBuilder
var configureServicesBuilder = StartupLoader.FindConfigureServicesDelegate(startupType, context.HostingEnvironment.EnvironmentName);
//调用Build方法返回ConfigureServices委托
var configureServices = configureServicesBuilder.Build(instance);
//传递services对象即IServiceCollection对象调用ConfigureServices方法
configureServices(services);

internal static ConfigureServicesBuilder FindConfigureServicesDelegate(Type startupType, string environmentName)
{
  //根据startupType和根据environmentName构建的Configure{0}Services字符串先去查找返回类型为IServiceProvider的方法
  //找不到在查找返回值为void类型的方法
  var servicesMethod = FindMethod(startupType, "Configure{0}Services", environmentName, typeof(IServiceProvider), required: false)
    "Configure{0}Services", environmentName, typeof(void), required: false);
  //根据查找的到的MethodInfo去构建ConfigureServicesBuilder实例
  return new ConfigureServicesBuilder(servicesMethod);
}

private static MethodInfo FindMethod(Type startupType, string methodName, string environmentName, Type returnType = null, bool required = true)
{
  //包含环境变量的ConfigureServices方法名称比如(ConfigureDevelopmentServices)
  var methodNameWithEnv = string.Format(CultureInfo.InvariantCulture, methodName, environmentName);
  //名为ConfigureServices的方法
  var methodNameWithNoEnv = string.Format(CultureInfo.InvariantCulture, methodName, "");
  //方法是共有的静态的或非静态的方法
  var methods = startupType.GetMethods(BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static);
  //查找包含环境变量的ConfigureServices方法名称
  var selectedMethods = methods.Where(method => method.Name.Equals(methodNameWithEnv, StringComparison.OrdinalIgnoreCase)).ToList();
  if (selectedMethods.Count > 1)
  {
    //找打多个满足规则的方法直接抛出异常
    throw new InvalidOperationException(string.Format("Having multiple overloads of method '{0}' is not supported.", methodNameWithEnv));

  }
  //如果不存在包含环境变量的ConfigureServices的方法比如(ConfigureDevelopmentServices)，则直接查找方法名为ConfigureServices的方法
  if (selectedMethods.Count == 0)
  {
    selectedMethods = methods.Where(method => method.Name.Equals(methodNameWithNoEnv, StringComparison.OrdinalIgnoreCase)).ToList();
    //如果存在多个则同样抛出异常
    if (selectedMethods.Count > 1)
    {
      throw new InvalidOperationException(string.Format("Having multiple overloads of method '{0}' is not supported.", methodNameWithNoEnv));
    }
  }

  var methodInfo = selectedMethods.FirstOrDefault();
  //如果没找到满足规则的方法，并且满足required参数，则抛出未找到方法的异常
  if (methodInfo == null)
  {
    if (required)
    {
      throw new InvalidOperationException(string.Format("A public method named '{0}' or '{1}' could not be found in the '{2}' type.",
        methodNameWithEnv,
        methodNameWithNoEnv,
        startupType.FullName));

    }
    return null;
  }
  //如果找到了名称一致的方法，但是返回类型和预期的不一致，也抛出异常
  if (returnType != null && methodInfo.ReturnType != returnType)
  {
    if (required)
    {
      throw new InvalidOperationException(string.Format("The '{0}' method in the type '{1}' must have a return type of '{2}'.",
        methodInfo.Name,
        startupType.FullName,
        returnType.Name));
    }
    return null;
  }
  return methodInfo;
}

internal class ConfigureServicesBuilder
{
  //构造函数传递的configureServices的MethodInfo
  public ConfigureServicesBuilder(MethodInfo configureServices)
  {
    MethodInfo = configureServices;
  }

  public MethodInfo MethodInfo { get; }
  public Func<Func<IServiceCollection, IServiceProvider>, Func<IServiceCollection, IServiceProvider StartupServiceFilters { get; set; } = f => f;
  //Build委托
  public Func<IServiceCollection, IServiceProvider> Build(object instance) => services => Invoke(instance, services);
  private IServiceProvider Invoke(object instance, IServiceCollection services)
  {
    //执行StartupServiceFilters委托参数为Func<IServiceCollection, IServiceProvider>类型的委托方法即Startup
    //返回了Func<IServiceCollection, IServiceProvider>委托，执行这个委托需传递services即IServiceCollections实例返回IServiceProvider类型
    return StartupServiceFilters(Startup)(services);
    IServiceProvider Startup(IServiceCollection serviceCollection) => InvokeCore(instance, serviceCollection);
  }

  private IServiceProvider InvokeCore(object instance, IServiceCollection services)
  {
    if (MethodInfo == null)
    {
      return null;
    }
    // 如果ConfigureServices方法包含多个参数或方法参数类型不是IServiceCollection类型则直接抛出异常
    // 也就是说ConfigureServices只能包含一个参数且类型为IServiceCollection
    var parameters = MethodInfo.GetParameters();
    if (parameters.Length > 1 ||
      parameters.Any(p => p.ParameterType != typeof(IServiceCollection)))
    {
      throw new InvalidOperationException("The ConfigureServices method must either be parameterless or take only one parameter of type IServiceCollection.");
    }
    //找到ConfigureServices方法的参数，并将services即IServiceCollection的实例传递给这个参数
    var arguments = new object[MethodInfo.GetParameters().Length];
    if (parameters.Length > 0)
    {
      arguments[0] = services;
    }
    // 执行返回IServiceProvider实例
    return MethodInfo.InvokeWithoutWrappingExceptions(instance, arguments) as IServiceProvider;
  }
}

//和ConfigureServices查找方式类似传递Startup实例和环境变量
ConfigureBuilder configureBuilder = StartupLoader.FindConfigureDelegate(startupType, context.HostingEnvironment.EnvironmentName);
services.Configure<GenericWebHostServiceOptions>(options =>
{
  //通过查看GenericWebHostServiceOptions的源码可知app其实就是IApplicationBuilder实例
  options.ConfigureApplication = app =>
  {
    startupError"https://github.com/dotnet/aspnetcore/blob/v3.1.9/src/Hosting/Hosting/src/Internal/StartupLoader.cs" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >点击查看源码"htmlcode">

internal static ConfigureBuilder FindConfigureDelegate(Type startupType, string environmentName)
{
  //通过startup类型和方法名为Configure或Configure+环境变量名称的方法
  var configureMethod = FindMethod(startupType, "Configure{0}", environmentName, typeof(void), required: true);
  //用查找到的方法去初始化ConfigureBuilder
  return new ConfigureBuilder(configureMethod);
}


从这里我们可以看到FindConfigureDelegate方法也是调用的FindMethod方法，只是传递的方法名字符串为Configure或Configure+环境变量，关于FindMethod的方法实现我们在上面讲解ConfigureServices方法的时候已经非常详细的说过了，这里就不过多的讲解了。总之是通过FindMethod去查找名为Configure的方法或名为Configure+环境变量的方法比如ConfigureDevelopment查找规则和ConfigureServices是完全一致的。但是Configure方法却可以通过参数注入注册到IServiceCollection中的服务，答案我们同样要在ConfigureBuilder类中去探寻

[点击查看源码"htmlcode">

internal class ConfigureBuilder
{
  //构造函数传递Configure的MethodInfo
  public ConfigureBuilder(MethodInfo configure)
  {
    MethodInfo = configure;
  }

  public MethodInfo MethodInfo { get; }
  //Build方法返回Action<IApplicationBuilder>委托
  public Action<IApplicationBuilder> Build(object instance) => builder => Invoke(instance, builder);
  //执行逻辑
  private void Invoke(object instance, IApplicationBuilder builder)
  {
    //通过IApplicationBuilder的ApplicationServices获取IServiceProvider实例创建一个作用域
    using (var scope = builder.ApplicationServices.CreateScope())
    {
      //获取IServiceProvider实例
      var serviceProvider = scope.ServiceProvider;
      //获取Configure的所有参数
      var parameterInfos = MethodInfo.GetParameters();
      var parameters = new object[parameterInfos.Length];
      for (var index = 0; index < parameterInfos.Length; index++)
      {
        var parameterInfo = parameterInfos[index];
        //如果方法参数为IApplicationBuilder类型则直接将传递过来的IApplicationBuilder赋值给它
        if (parameterInfo.ParameterType == typeof(IApplicationBuilder))
        {
          parameters[index] = builder;
        }
        else
        {
          try
          {
            //根据方法的参数类型在serviceProvider中获取具体实例赋值给对应参数
            parameters[index] = serviceProvider.GetRequiredService(parameterInfo.ParameterType);
          }
          catch (Exception ex)
          {
            //如果对应的方法参数名称，没在serviceProvider中获取到则直接抛出异常
            //变相的说明了Configure方法的参数必须是注册在IServiceCollection中的
          }
        }
      }
      MethodInfo.InvokeWithoutWrappingExceptions(instance, parameters);
    }
  }
}


通过ConfigureBuilder类的实现逻辑，可以清晰的看到为何Configure方法参数可以注入任何在IServiceCollection中注册的服务了。接下来我们总结一下Configure方法的初始化逻辑，首先在Startup中查找方法名为Configure或Configure+环境变量名称(比如ConfigureDevelopment)的方法，然后查找IApplicationBuilder类型的参数，如果找到则将程序中的IApplicationBuilder实例传递给它。至于为何Configure方法能够通过参数注入任何在IServiceCollection中注册的服务，则是因为循环Configure中的所有参数然后在IOC容器中获取对应实例赋值过来，Configure方法的参数一定得是在IServiceCollection注册过的类型，否则会抛出异常。
ConfigureContainer为何会被调用
如果你在ASP.NET Core 3.1中使用过Autofac那么你对ConfigureContainer方法一定不陌生，它和ConfigureServices、Configure方法一样的神奇，在几乎没有任何约束的情况下我们只需要定义ConfigureContainer方法并为方法传递一个ContainerBuilder参数，那么这个方法就能顺利的被调用了。这一切究竟是如何实现的呢，接下来我们继续探究源码，找到了如下的逻辑

[点击查看源码"htmlcode">

//根据规则查找最终返回ConfigureContainerBuilder实例
var configureContainerBuilder = StartupLoader.FindConfigureContainerDelegate(startupType, context.HostingEnvironment.EnvironmentName);
if (configureContainerBuilder.MethodInfo != null)
{
  //获取容器类型比如如果是autofac则类型为ContainerBuilder
  var containerType = configureContainerBuilder.GetContainerType();
  // 存储configureContainerBuilder实例
  _builder.Properties[typeof(ConfigureContainerBuilder)] = configureContainerBuilder;
  //构建一个Action<HostBuilderContext,containerType>类型的委托
  var actionType = typeof(Action<,>).MakeGenericType(typeof(HostBuilderContext), containerType);

  // 获取此类型的私有ConfigureContainer方法，然后声明该方法的泛型为容器类型，然后创建这个方法的委托
  var configureCallback = GetType().GetMethod(nameof(ConfigureContainer), BindingFlags.NonPublic | BindingFlags.Instance)
                   .MakeGenericMethod(containerType)
                   .CreateDelegate(actionType, this);

  // 等同于执行_builder.ConfigureContainer<T>(ConfigureContainer)，其中T为容器类型。
  //C onfigureContainer表示一个委托,即我们在Startup中定义的ConfigureContainer委托
  typeof(IHostBuilder).GetMethods().First(m => m.Name == nameof(IHostBuilder.ConfigureContainer))
    .MakeGenericMethod(containerType)
    .InvokeWithoutWrappingExceptions(_builder, new object[] { configureCallback });
}


继续使用老配方，我们查看StartupLoader的FindConfigureContainerDelegate方法实现

[点击查看源码"htmlcode">

internal static ConfigureContainerBuilder FindConfigureContainerDelegate(Type startupType, string environmentName)
{
  //根据startupType和根据environmentName构建的Configure{0}Services字符串先去查找返回类型为IServiceProvider的方法
  var configureMethod = FindMethod(startupType, "Configure{0}Container", environmentName, typeof(void), required: false);
  //用查找到的方法去初始化ConfigureContainerBuilder
  return new ConfigureContainerBuilder(configureMethod);
}


果然还是这个配方这个味道，废话不多说直接查看ConfigureContainerBuilder源码

[点击查看源码"htmlcode">

internal class ConfigureContainerBuilder
{
  public ConfigureContainerBuilder(MethodInfo configureContainerMethod)
  {
    MethodInfo = configureContainerMethod;
  }
  public MethodInfo MethodInfo { get; }
  public Func<Action<object>, Action<object ConfigureContainerFilters { get; set; } = f => f;
  public Action<object> Build(object instance) => container => Invoke(instance, container);
  //查找容器类型，其实就是ConfigureContainer方法的的唯一参数
  public Type GetContainerType()
  {
    var parameters = MethodInfo.GetParameters();
    //ConfigureContainer方法只能包含一个参数
    if (parameters.Length != 1)
    {
      throw new InvalidOperationException($"The {MethodInfo.Name} method must take only one parameter.");
    }
    return parameters[0].ParameterType;
  }

  private void Invoke(object instance, object container)
  {
    ConfigureContainerFilters(StartupConfigureContainer)(container);
    void StartupConfigureContainer(object containerBuilder) => InvokeCore(instance, containerBuilder);
  }
  
  //根据传递的container对象执行ConfigureContainer方法逻辑比如使用autofac时ConfigureContainer(ContainerBuilder)
  private void InvokeCore(object instance, object container)
  {
    if (MethodInfo == null)
    {
      return;
    }
    var arguments = new object[1] { container };
    MethodInfo.InvokeWithoutWrappingExceptions(instance, arguments);
  }
}


果不其然千年老方下来还是那个味道，和ConfigureServices、Configure方法思路几乎一致。这里需要注意的是GetContainerType获取的容器类型是ConfigureContainer方法的唯一参数即容器类型，如果传递多个参数则直接抛出异常。其实Startup的ConfigureContainer方法经过花里胡哨的一番操作之后，最终还是转换成了雷士如下的操作方式,这个我们在上面代码中构建actionType的时候就可以看出，最终通过查找到的容器类型去完成注册等相关操作，这里就不过多的讲解了


Host.CreateDefaultBuilder(args)
    .ConfigureContainer<ContainerBuilder>((context,container)=> {
      container.RegisterType<PersonService>().As<IPersonService>().InstancePerLifetimeScope();
    });


总结
本篇文章我们主要是围绕着Startup是如何被初始化进行讲解的，分别讲解了Startup是如何被实例化的，为何Startup的构造函数只能传递IWebHostEnvironment、IHostEnvironment、IConfiguration类型的参数，以及ConfigureServices、Configure、ConfigureContainer方法是如何查找到并被初始化调用的。其中虽然涉及到的代码比较多，但是整体思路在阅读源码后还是比较清晰的。由于笔者文笔有限，可能许多地方描述的不够清晰，亦或是本人能力有限理解的不够透彻，不过本人在文章中都标记了源码所在位置的链接，如果有感兴趣的同学可以自行点击连接查看源码。Startup类比较常用，如果能够更深层次的了解其原理，对我们实际编程过程中会有很大的帮助，同时呼吁更多的小伙伴们深入阅读了解.NET Core的源码并分享出来。如有各位有疑问或者有了解的更透彻的，欢迎评论区提问或批评指导。