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

標題相當難取，內容也許和您想的不一樣，而且網上已經有很多這方面的資料了，我不過是在實踐過程中作下記錄。廢話少說，直接開始。

Exception

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當服務端拋出未處理異常時，most exceptions are translated into an HTTP response with status code 500, Internal Server Error.當然我們也可以拋出一個特殊的異常HttpResponseException，它將被直接寫入響應流，而不會被轉成500。

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

有時要對服務端異常做一封裝，以便對客戶端隱藏具體細節，或者統一格式，那么可創建一繼承自System.Web.Http.Filters.ExceptionFilterAttribute的特性，如下：

public class APIExceptionFilterAttribute : ExceptionFilterAttribute {public override void OnException(HttpActionExecutedContext context){//業務異常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);}//其它異常else{context.Response = new HttpResponseMessage { StatusCode = System.Net.HttpStatusCode.InternalServerError };}} }

然后將該Attribute應用到action或controller，或者GlobalConfiguration.Configuration.Filters.Add(new?APIExceptionFilterAttribute());使之應用于所有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());）。當然，在上述代碼中，我們也可以在OnException方法中直接拋出HttpResponseException，效果是一樣的。

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.

如果要返回給客戶端的不僅僅是一串字符串，比如是json對象，那么可以使用HttpError這個類。

以上知識主要來自Exception Handling in ASP.NET Web API。

ActionFilterAttribute、ApiControllerActionInvoker?

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有時要在action執行前后做額外處理，那么ActionFilterAttribute和ApiControllerActionInvoker就派上用場了。比如客戶端請求發過來的參數為用戶令牌字符串token，我們要在action執行之前先將其轉為action參數列表中對應的用戶編號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)){//參數列表中不存在userid，寫入日志//……var response = new HttpResponseMessage();response.Content = new StringContent("用戶信息轉換異常.");response.StatusCode = HttpStatusCode.Conflict;//在這里為了不繼續走流程，要throw出來，才會立馬返回到客戶端throw new HttpResponseException(response);}//userid系統賦值actionContext.ActionArguments[UserID] = actionContext.Request.Properties["shumi_userid"];base.OnActionExecuting(actionContext);}public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext){base.OnActionExecuted(actionExecutedContext);} }

ActionFilterAttribute如何應用到action，和前面的ExceptionFilterAttribute類似。

ApiControllerActionInvoker以上述Exception為例：

public class ServerAPIControllerActionInvoker : ApiControllerActionInvoker {public override Task<HttpResponseMessage> InvokeActionAsync(HttpActionContext actionContext, CancellationToken cancellationToken){//對actionContext做一些預處理//……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;} }

然后注冊至GlobalConfiguration.Configuration.Services中。由于ApiControllerActionInvoker乃是影響全局的，所以若要對部分action進行包裝處理，應該優先選擇ActionFilterAttribute。?

DelegatingHandler

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前面的攔截都發生在請求已被路由至對應的action后發生，有一些情況需要在路由之前就做預先處理，或是在響應流返回過程中做后續處理，這時我們就要用到DelegatingHandler。比如對請求方的身份驗證，當驗證未通過時直接返回錯誤信息，否則進行后續調用。

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("請求參數有重復.", HttpStatusCode.BadRequest);}}//請求方身份驗證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);} }

?參考資料：

• ASP.NET Web API Exception Handling
• Implementing message handlers to track your ASP.NET Web API usage
• MVC4 WebAPI(二)——Web API工作方式
• Asp.Net MVC及Web API框架配置會碰到的幾個問題及解決方案

轉載請注明原文出處：http://www.cnblogs.com/newton/p/3238082.html

轉載于:https://www.cnblogs.com/smileberry/p/7093326.html

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