跨域资源共享（CORS）在ASP.NET Web API中是如何实现的？

在《通过扩展让ASP.NET Web API支持W3C的CORS规范》中，我们通过自定义的HttpMessageHandler自行为ASP.NET Web API实现了针对CORS的支持，实际上ASP.NET Web API自身也是这么做的，该自定义HttpMessageHandler就是System.Web.Http.Cors.CorsMessageHandler。

class CorsMessageHandler : DelegatingHandler

   2: {   

public CorsMessageHandler(HttpConfiguration httpConfiguration);

override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken);

   5:  

virtual Task<HttpResponseMessage> HandleCorsPreflightRequestAsync(HttpRequestMessage request, CorsRequestContext corsRequestContext, CancellationToken cancellationToken);   

virtual Task<HttpResponseMessage> HandleCorsRequestAsync(HttpRequestMessage request, CorsRequestContext corsRequestContext, CancellationToken cancellationToken);

   8: }