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.
.net还内置了HttpError这个类，若想返回格式化对象（如json、xml等），用起来更方便。The HttpError class is actually a key-value collection (it derives from Dictionary<string, object>), so you can add your own key-value pairs.
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。另外ApiControllerActionInvoker在ActionFilterAttribute之前处理。
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); } }
这里的DelegatingHandler用于服务端，其实DelegatingHandler也可以在发起调用时使用，HttpClient可接收一个DelegatingHandler作为消息处理器。
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