$.ajax 跨域请求 Web Api

      WepApi确实方便好用，没有配置文件，一个apicontroller直接可以干活了。但今天用$.ajax跨域请求的时候总是获取不到数据，用 fiddler一看确实抓到了数据，但回到$.ajax函数中，直接触发了error，没有触发success，即使状态码是200。用 apiclient或者浏览器直接访问都是ok的。搜罗一番。最终在这篇文章上面找到答案 。http://code.msdn.microsoft.com/windowsdesktop/Implementing-CORS-support-a677ab5d

原因

     在默认情况下，为防止CSRF跨站伪造攻击，一个网页从另外一个域的网页获取数据的时候就会受到限制。有一些方法可以突破这个限制，JSONP就是其一。它使用<script> 标签加一个回调函数。但JSONP 只支持Get方法。而CORS(Cross-Origin Resource Sharing) 跨域资源共享，是一种新的header规范，可以让服务器端放松跨域的限制，可以根据header来切换限制或不限制跨域请求。它支持所有的Http请求 方式。跨域的资源请求带有一个Http header：Origin，如果服务器支持CORS，响应就会带有一个header:Access-Control-Allow-Origin ,也有一些特殊的请求。采用 HTTP “OPTIONS” 的方式，hearder中带有Access-Control-Request-Method或Access-Control-Request- Headers，服务器响应的hearder中需要带有Access-Control-Allow-Methods，Access-Control- Allow-Headers才行。

实现 

    那怎么实现CORS呢，这用到了Message Handler。它可以在管道中拦截并修改Request，代码如下：

 

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public class CorsHandler : DelegatingHandler    { const string Origin = "Origin"; const string AccessControlRequestMethod = "Access-Control-Request-Method"; const string AccessControlRequestHeaders = "Access-Control-Request-Headers"; const string AccessControlAllowOrigin = "Access-Control-Allow-Origin"; const string AccessControlAllowMethods = "Access-Control-Allow-Methods"; const string AccessControlAllowHeaders = "Access-Control-Allow-Headers"; protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)        { bool isCorsRequest = request.Headers.Contains(Origin); bool isPreflightRequest = request.Method == HttpMethod.Options; if (isCorsRequest)            { if (isPreflightRequest)                { return Task.Factory.StartNew<HttpResponseMessage>(() => {                        HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);                        response.Headers.Add(AccessControlAllowOrigin, request.Headers.GetValues(Origin).First()); string accessControlRequestMethod = request.Headers.GetValues(AccessControlRequestMethod).FirstOrDefault(); if (accessControlRequestMethod != null)                        {                            response.Headers.Add(AccessControlAllowMethods, accessControlRequestMethod);                        } string requestedHeaders = string.Join(", ", request.Headers.GetValues(AccessControlRequestHeaders)); if (!string.IsNullOrEmpty(requestedHeaders))                        {                            response.Headers.Add(AccessControlAllowHeaders, requestedHeaders);                        } return response;                    }, cancellationToken);                } else { return base.SendAsync(request, cancellationToken).ContinueWith<HttpResponseMessage>(t => {                        HttpResponseMessage resp = t.Result;                        resp.Headers.Add(AccessControlAllowOrigin, request.Headers.GetValues(Origin).First()); return resp;                    });                }            } else { return base.SendAsync(request, cancellationToken);            }        }    }

然后在Global中加入：

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protected void Application_Start(object sender, EventArgs e)        {  GlobalConfiguration.Configuration.MessageHandlers.Add(new CorsHandler()); WebApiConfig.Register(GlobalConfiguration.Configuration);        }

脚本：

 

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$.ajax({ // url: "http://localhost:11576/api/Values", url: "http://localhost:39959/api/user/login?name=niqiu&pwd=123456",               type: "GET", //contentType: "application/json;", success: function(result) {                    alert(result.status);                },                error: function (XMLHttpRequest, textStatus, errorThrown) {                    alert("出错！XMLHttpRequest:" + XMLHttpRequest.status);                }            });