c# - 给定一个 ContainerBuilder，我可以注册一个缺失的依赖处理程序吗？

Question

我正在尝试创建一个具有以下签名的方法:

void Chain(ContainerBuilder builder, IServiceProvider fallbackServiceProvider)
{
   // ...
}

这个想法是Chain可以按如下方式使用:

IServiceProvider fallbackProvider = someExternalProvider;
var builder = new ContainerBuilder();

// Custom registration might happen before and/or after the call to Chain
builder.Register<MyCustomService>().As<IMyCustomService>();
builder.Register<MyExternalServiceReplacement>.As<IExternalService>();

Chain(builder, someExternalProvider);

IContainer container = builder.Build();

// customService should be a MyCustomService
var customService = container.Resolve<IMyCustomService>();

// replacedService should be overridden by MyExternalServiceReplacement
// even though an IExternalService also exists in someExternalProvider
var replacedService = container.Resolve<IExternalService>();

// nonReplacedService should come from someExternalProvider since
// no IExternalService2 was registered with the ContainerBuilder
var nonReplacedService = container.Resolve<IExternalService2>();

理想情况下，我可以向 ContainerBuilder 注册某种类型的缺失依赖项处理程序。

或者，我可能会通过某种方式来注册一个组件，该组件可以拦截对 Resolve*、TryResolve* 等的每次调用...这也需要拦截构造函数注入(inject)的依赖解析。

遗憾的是，无法查询 IServiceProvider 以获取它提供的每项服务。我只能调用 fallbackServicProvider 的 object IServiceProvider.GetService(Type serviceType) 方法。

Answer 1

您需要一个自定义的IRegistrationSource 实现:当容器需要提供服务时，它会查询已注册的注册源以获取任何可用的实现。

因此在注册源中，您可以要求您的 IServiceProvider 为您提供给定类型的回退实现。

这里有一篇很好的文章介绍了 Autofac 中的整个注册源:Declarative Context Adapters in Autofac 2

基于此，我拼凑了一个原型(prototype) IRegistrationSource 实现(因此它没有经过全面测试，也没有准备好生产，但它可以与您的示例场景一起使用)什么你可以建立在:

public class MyRegistrationSource : IRegistrationSource
{
    private readonly IServiceProvider serviceProvider;

    public MyRegistrationSource(IServiceProvider serviceProvider)
    {
        this.serviceProvider = serviceProvider;
    }

    public IEnumerable<IComponentRegistration> RegistrationsFor(Service service,
        Func<Service, IEnumerable<IComponentRegistration>> registrationAccessor)
    {
        // there are other registration exists in the container
        if (registrationAccessor(service).Any())
            return Enumerable.Empty<IComponentRegistration>();

        var swt = service as IServiceWithType;
        if (swt == null)
            return Enumerable.Empty<IComponentRegistration>();

        // try to get an instance from the IServiceProvider
        var instance = serviceProvider.GetService(swt.ServiceType);
        if (instance == null)
            return Enumerable.Empty<IComponentRegistration>();

        // register the instance in the container
        return new[]
            {
                RegistrationBuilder.ForDelegate(swt.ServiceType, 
                    (c, p) => instance)
                    .CreateRegistration()
            };
    }
    public bool IsAdapterForIndividualComponents { get { return false; } }
}

你可以这样使用它:

 var builder = new ContainerBuilder();

 // Custom registration might happen before and/or after the call to Chain
 builder.RegisterType<MyCustomService>().As<IMyCustomService>();
 builder.RegisterType<MyExternalServiceReplacement>().As<IExternalService>();

 //Chain(builder, someExternalProvider);
 builder.RegisterSource(new MyRegistrationSource(new ServiceProvider()));

 IContainer container = builder.Build();