ASP.NET Web API之消息[拦截]处理（转）

出处：http://www.cnblogs.com/Leo_wl/p/3238719.html

标题相当难取，内容也许和您想的不一样，而且网上已经有很多这方面的资料了，我不过是在实践过程中作下记录。废话少说，直接开始。

Exception

当服务端抛出未处理异常时，most exceptions are translated into an HTTP response with status code 500, Internal Server Error.当然我们也可以抛出一个特殊的异常HttpResponseException，它将被直接写入响应流，而不会被转成500。

public Product GetProduct(int id)
{Product item = repository.Get(id);if (item == null){throw new HttpResponseException(HttpStatusCode.NotFound);}return item;
}

有时要对服务端异常做一封装，以便对客户端隐藏具体细节，或者统一格式，那么可创建一继承自System.Web.Http.Filters.ExceptionFilterAttribute的特性，如下：

public class APIExceptionFilterAttribute : ExceptionFilterAttribute
{public override void OnException(HttpActionExecutedContext context){//业务异常if (context.Exception is BusinessException){context.Response = new HttpResponseMessage { StatusCode = System.Net.HttpStatusCode.ExpectationFailed };BusinessException exception = (BusinessException)context.Exception;context.Response.Headers.Add("BusinessExceptionCode", exception.Code);context.Response.Headers.Add("BusinessExceptionMessage", exception.Message);}//其它异常else{context.Response = new HttpResponseMessage { StatusCode = System.Net.HttpStatusCode.InternalServerError };}}
}

然后将该Attribute应用到action或controller，或者GlobalConfiguration.Configuration.Filters.Add(new APIExceptionFilterAttribute());使之应用于所有action（If you use the "ASP.NET MVC 4 Web Application" project template to create your project, put your Web API configuration code inside the WebApiConfig class, which is located in the App_Start folder:config.Filters.Add(newProductStore.NotImplExceptionFilterAttribute());）。当然，在上述代码中，我们也可以在OnException方法中直接抛出HttpResponseException，效果是一样的。

Note: Something to have in mind is that the ExceptionFilterAttribute will be ignored if the ApiController action method throws a HttpResponseException；If something goes wrong in the ExceptionFilterAttribute and an exception is thrown that is not of type HttpResponseException, a formatted exception will be thrown with stack trace etc to the client.

如果要返回给客户端的不仅仅是一串字符串，比如是json对象，那么可以使用HttpError这个类。

以上知识主要来自Exception Handling in ASP.NET Web API。

ActionFilterAttribute、ApiControllerActionInvoker

有时要在action执行前后做额外处理，那么ActionFilterAttribute和ApiControllerActionInvoker就派上用场了。比如客户端请求发过来的参数为用户令牌字符串token，我们要在action执行之前先将其转为action参数列表中对应的用户编号ID，如下：

public class TokenProjectorAttribute : ActionFilterAttribute
{private string _userid = "userid";public string UserID{get { return _userid; }set { _userid = value; }}public override void OnActionExecuting(HttpActionContext actionContext){if (!actionContext.ActionArguments.ContainsKey(UserID)){//参数列表中不存在userid，写入日志//……var response = new HttpResponseMessage();response.Content = new StringContent("用户信息转换异常.");response.StatusCode = HttpStatusCode.Conflict;//在这里为了不继续走流程，要throw出来，才会立马返回到客户端throw new HttpResponseException(response);}//userid系统赋值actionContext.ActionArguments[UserID] = actionContext.Request.Properties["shumi_userid"];base.OnActionExecuting(actionContext);}public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext){base.OnActionExecuted(actionExecutedContext);}
}

ActionFilterAttribute如何应用到action，和前面的ExceptionFilterAttribute类似。

ApiControllerActionInvoker以上述Exception为例：

public class ServerAPIControllerActionInvoker : ApiControllerActionInvoker
{public override Task<HttpResponseMessage> InvokeActionAsync(HttpActionContext actionContext, CancellationToken cancellationToken){//对actionContext做一些预处理//……var result = base.InvokeActionAsync(actionContext, cancellationToken);if (result.Exception != null && result.Exception.GetBaseException() != null){var baseException = result.Exception.GetBaseException();if (baseException is BusinessException){return Task.Run<HttpResponseMessage>(() =>{var response = new HttpResponseMessage(HttpStatusCode.ExpectationFailed);BusinessException exception = (BusinessException)baseException;response.Headers.Add("BusinessExceptionCode", exception.Code);response.Headers.Add("BusinessExceptionMessage", exception.Message);return response;});}else{return Task.Run<HttpResponseMessage>(() => new HttpResponseMessage(HttpStatusCode.InternalServerError));}}return result;}
}

然后注册至GlobalConfiguration.Configuration.Services中。由于ApiControllerActionInvoker乃是影响全局的，所以若要对部分action进行包装处理，应该优先选择ActionFilterAttribute。

DelegatingHandler

前面的拦截都发生在请求已被路由至对应的action后发生，有一些情况需要在路由之前就做预先处理，或是在响应流返回过程中做后续处理，这时我们就要用到DelegatingHandler。比如对请求方的身份验证，当验证未通过时直接返回错误信息，否则进行后续调用。

public class AuthorizeHandler : DelegatingHandler
{private static IAuthorizer _authorizer = null;static AuthorizeHandler(){ }protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken){if (request.Method == HttpMethod.Post){var querystring = HttpUtility.ParseQueryString(request.RequestUri.Query);var formdata = request.Content.ReadAsFormDataAsync().Result;if (querystring.AllKeys.Intersect(formdata.AllKeys).Count() > 0){return SendError("请求参数有重复.", HttpStatusCode.BadRequest);}}//请求方身份验证AuthResult result = _authorizer.AuthRequest(request);if (!result.Flag){return SendError(result.Message, HttpStatusCode.Unauthorized);}request.Properties.Add("shumi_userid", result.UserID);return base.SendAsync(request, cancellationToken);}private Task<HttpResponseMessage> SendError(string error, HttpStatusCode code){var response = new HttpResponseMessage();response.Content = new StringContent(error);response.StatusCode = code;return Task<HttpResponseMessage>.Factory.StartNew(() => response);}
}

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