装饰 ASP.NET Web API IHttpController

Question

我正在尝试用装饰器包装 Web API 控制器（IHttpController 实现），但是当我这样做时，Web API 会引发异常，因为它以某种方式期待实际的实现。

将装饰器应用于控制器是我成功应用于 MVC 控制器的一个技巧，我显然喜欢在 Web API 中做同样的事情。

我创建了一个自定义的IHttpControllerActivator，它允许解析修饰的IHttpController 实现。这是一个剥离的实现：

public class CrossCuttingConcernHttpControllerActivator : IHttpControllerActivator {
    private readonly Container container;
    public CrossCuttingConcernHttpControllerActivator(Container container) {
        this.container = container;
    }

    public IHttpController Create(HttpRequestMessage request, 
        HttpControllerDescriptor controllerDescriptor, Type controllerType)
    {
        var controller = (IHttpController)this.container.GetInstance(controllerType);

        // Wrap the instance in one or multiple decorators. Note that in reality, the 
        // decorator is applied by the container, but that doesn't really matter here.
        return new MyHttpControllerDecorator(controller);
    }
}

我的装饰器看起来像这样：

public class MyHttpControllerDecorator : IHttpController {
    private readonly IHttpController decoratee;
    public MyHttpControllerDecorator(IHttpController decoratee) {
        this.decoratee = decoratee;
    }

    public Task<HttpResponseMessage> ExecuteAsync(
        HttpControllerContext controllerContext,
        CancellationToken cancellationToken)
    {
        // this decorator does not add any logic. Just the minimal amount of code to
        // reproduce the issue.
        return this.decoratee.ExecuteAsync(controllerContext, cancellationToken);
    }
}

但是，当我运行我的应用程序并请求 ValuesController 时，Web API 会向我抛出以下 InvalidCastException：

无法转换类型为“WebApiTest.MyHttpControllerDecorator”的对象 输入“WebApiTest.Controllers.ValuesController”。

堆栈跟踪：

at lambda_method(Closure , Object , Object[] )
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass13.<GetExecutor>b__c(Object instance, Object[] methodParameters)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object instance, Object[] arguments)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.<>c__DisplayClass5.<ExecuteAsync>b__4()
at System.Threading.Tasks.TaskHelpers.RunSynchronously[TResult](Func`1 func, CancellationToken cancellationToken)

就好像 Web API 为我们提供了 IHttpController 抽象但跳过它并仍然依赖于实现本身。这当然会严重违反依赖倒置原则，并使抽象完全无用。所以我可能做错了什么。

我做错了什么？如何愉快地装饰我的 API 控制器？

Answer 1

您可以提供IHttpControllerSelector 的自定义实现来更改为特定控制器实例化的类型。 （请注意，我没有对此进行过彻底的测试）

将装饰器更新为通用

public class MyHttpControllerDecorator<T> : MyHttpController
    where T : MyHttpController
{
    public readonly T decoratee;

    public MyHttpControllerDecorator(T decoratee)
    {
        this.decoratee = decoratee;
    }

    public Task<HttpResponseMessage> ExecuteAsync(
        HttpControllerContext controllerContext,
        CancellationToken cancellationToken)
    {
        return this.decoratee.ExecuteAsync(controllerContext, cancellationToken);
    }

    [ActionName("Default")]
    public DtoModel Get(int id)
    {
        return this.decoratee.Get(id);
    }
}

定义IHttpControllerSelector的自定义实现

public class CustomControllerSelector : DefaultHttpControllerSelector
{
    private readonly HttpConfiguration configuration;
    public CustomControllerSelector(HttpConfiguration configuration)
        : base(configuration)
    {
        this.configuration = configuration;
    }

    public override HttpControllerDescriptor SelectController(
        HttpRequestMessage request)
    {
        var controllerTypes = this.configuration.Services
            .GetHttpControllerTypeResolver().GetControllerTypes(
                this.configuration.Services.GetAssembliesResolver());

        var matchedTypes = controllerTypes.Where(i => 
             typeof(IHttpController).IsAssignableFrom(i)).ToList();

        var controllerName = base.GetControllerName(request);
        var matchedController = matchedTypes.FirstOrDefault(i => 
                i.Name.ToLower() == controllerName.ToLower() + "controller");

        if (matchedController.Namespace == "WebApiTest.Controllers")
        {
            Type decoratorType = typeof(MyHttpControllerDecorator<>);
            Type decoratedType = decoratorType.MakeGenericType(matchedController);
            return new HttpControllerDescriptor(this.configuration, controllerName, decoratedType);
        }
        else
        {
            return new HttpControllerDescriptor(this.configuration, controllerName, matchedController);
        }
    }
}

注册控制器时，添加控制器类型的修饰版本的注册

var container = new SimpleInjector.Container();

var services = GlobalConfiguration.Configuration.Services;

var controllerTypes = services.GetHttpControllerTypeResolver()
    .GetControllerTypes(services.GetAssembliesResolver());

Type decoratorType = typeof(MyHttpControllerDecorator<>);
foreach (var controllerType in controllerTypes)
{
    if (controllerType.Namespace == "WebApiTest.Controllers")
    {
        Type decoratedType = decoratorType.MakeGenericType(controllerType);
        container.Register(decoratedType, () => 
            DecoratorBuilder(container.GetInstance(controllerType) as dynamic));
    }
    else
    {
        container.Register(controllerType);
    }
}

注册IHttpControllerSelector的实现

GlobalConfiguration.Configuration.Services.Replace(
    typeof(IHttpControllerSelector),
    new CustomControllerSelector(GlobalConfiguration.Configuration));

这是创建装饰实例的方法

private MyHttpControllerDecorator<T> DecoratorBuilder<T>(T instance)
    where T : IHttpController
{
    return new MyHttpControllerDecorator<T>(instance);
}

Answer 2

您可以通过实现IHttpActionInvoker 并在IHttpController 抽象不再相关时将装饰器“转换”为装饰实例来解决此问题。

这很容易通过从ApiControllerActionInvoker继承来完成。

（我已经对示例进行了硬编码，并且希望任何现实世界的实现都更加灵活。）

public class ContainerActionInvoker : ApiControllerActionInvoker
{
    private readonly Container container;

    public ContainerActionInvoker(Container container)
    {
        this.container = container;
    }

    public override Task<HttpResponseMessage> InvokeActionAsync(
        HttpActionContext actionContext, 
        CancellationToken cancellationToken)
    {
        if (actionContext.ControllerContext.Controller is MyHttpControllerDecorator)
        {
            MyHttpControllerDecorator decorator =
                (MyHttpControllerDecorator)actionContext.ControllerContext.Controller;
            // decoratee changed to public for the example
            actionContext.ControllerContext.Controller = decorator.decoratee;
        }

        var result = base.InvokeActionAsync(actionContext, cancellationToken);
        return result;
    }
}

这是在Global.asax.cs注册的

GlobalConfiguration.Configuration.Services.Replace(
    typeof(IHttpControllerActivator),
    new CrossCuttingConcernHttpControllerActivator(container));

GlobalConfiguration.Configuration.Services.Replace(
    typeof(IHttpActionInvoker),
    new ContainerActionInvoker(container)); 

您是否真的想要这样做是另一回事 - 谁知道更改 actionContext 的后果？

Answer 3

我想说，在 ASP.NET Web API 中实现这种行为的自然、设计方法是使用 Custom Message Handlers / Delegation Handlers

例如，我确实有这个DelegationHandler

public class AuthenticationDelegationHandler : DelegatingHandler
{
    protected override System.Threading.Tasks.Task<HttpResponseMessage> 
        SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // I. do some stuff to create Custom Principal
        // e.g.
        var principal = CreatePrincipal();
        ...

        // II. return execution to the framework            
        return base.SendAsync(request, cancellationToken).ContinueWith(t =>
        {
            HttpResponseMessage resp = t.Result;
            // III. do some stuff once finished
            // e.g.:
            // SetHeaders(resp, principal);

            return resp;
        });
    }

这就是如何将它注入到结构中：

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.MessageHandlers.Add(new AuthenticationDelegationHandler());