.NET Web API CORS PreFlight请求

创建一个派生自“DelegatingHandler”的类：

public class PreflightRequestsHandler : DelegatingHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken)
    {
        if (request.Headers.Contains("Origin") && request.Method.Method.Equals("OPTIONS"))
        {
            var response = new HttpResponseMessage { StatusCode = HttpStatusCode.OK };
            // Define and add values to variables: origins,headers,methods (can be global)               
            response.Headers.Add("Access-Control-Allow-Origin",origins);
            response.Headers.Add("Access-Control-Allow-Headers",headers);
            response.Headers.Add("Access-Control-Allow-Methods",methods);
            var tsc = new TaskCompletionSource<HttpResponseMessage>();
            tsc.SetResult(response);
            return tsc.Task;
        }
        return base.SendAsync(request,cancellationToken);
    }

}

后来在WebApiconfig.cs中的Register方法添加：

public static void Register(HttpConfiguration config)
{
    // Define and add values to variables: origins,methods (can be global) 
    // Enable global CORS
    config.EnableCors(new EnableCorsAttribute(origins,methods));

    // Add handler to deal with preflight requests,this is the important part
    config.MessageHandlers.Add(new PreflightRequestsHandler()); // Defined above
    .
    .
    .
}