An IHttpModule for Enabling CORS

CrossOriginSupportModule.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Sample
{
    public class CrossOriginRequestInfo
    {
        public string Origin { get; set; }
        public string Method { get; set; }
        public string Headers { get; set; }
    }

    public class CrossOriginSupportModule : IHttpModule
    {
        public const string ContextKey = "__crossOriginRequestInfo";

        public const string Options = "OPTIONS";
        public const string Origin = "Origin";
        public const string AccessControlRequestMethod = "Access-Control-Request-Method";
        public const string AccessControlRequestHeaders = "Access-Control-Request-Headers";

        public const string AccessControlAllowOrigin = "Access-Control-Allow-Origin";
        public const string AccessControlAllowMethods = "Access-Control-Allow-Methods";
        public const string AccessControlAllowHeaders = "Access-Control-Allow-Headers";
        public const string AccessControlAllowCredentials = "Access-Control-Allow-Credentials";
        public const string AccessControlMaxAage = "Access-Control-Max-Age";       

        public void Init(HttpApplication context)
        {
            context.BeginRequest += ContextOnBeginRequest;            
            context.PreSendRequestHeaders += ContextOnPreSendRequestHeaders;
        }

        private void ContextOnPreSendRequestHeaders(object sender, EventArgs eventArgs)
        {
            var application = (HttpApplication)sender;
            var context = application.Context;
            var response = context.Response;
            
            var info = context.Items[ContextKey] as CrossOriginRequestInfo;            
            if (info != null)
            {
                response.Headers[AccessControlAllowOrigin] = info.Origin;
            }
        }

        private void ContextOnBeginRequest(object sender, EventArgs eventArgs)
        {
            var application = (HttpApplication) sender;
            var context = application.Context;
            var request = context.Request;

            if (!String.IsNullOrEmpty(request.Headers[Origin]))
            {
                context.Items[ContextKey] = new CrossOriginRequestInfo
                {
                    Origin = request.Headers[Origin],
                    Method = request.Headers[AccessControlRequestMethod],
                    Headers = request.Headers[AccessControlRequestHeaders]
                };

                if (request.HttpMethod == Options) context.RewritePath("~/corsPreFlight");
            }            
        }
      
        public void Dispose()
        {
            
        }
    }

    public class CrossOriginSupportOptionsHandler : IHttpHandler
    {
        public bool IsReusable
        {
            get { return true; }
        }

        public void ProcessRequest(HttpContext context)
        {
            var response = context.Response;

            response.StatusCode = 200;
            response.ContentType = "text/plain";

            var info = context.Items[CrossOriginSupportModule.ContextKey] as CrossOriginRequestInfo;
            if (info != null)
            {  
                response.Headers[CrossOriginSupportModule.AccessControlAllowOrigin] = info.Origin;
                response.Headers[CrossOriginSupportModule.AccessControlAllowMethods] = info.Method;
                response.Headers[CrossOriginSupportModule.AccessControlAllowHeaders] = info.Headers;
                response.Headers[CrossOriginSupportModule.AccessControlMaxAage] = 1728000.ToString();                
            }

            response.End();
        }
    }
}

Web.config

<httpModules>
  <add name="CrossOriginSupportModule" type="Sample.CrossOriginSupportModule, Sample" />
</httpModules>
<httpHandlers>
  <add verb="*" path="/corsPreFlight" type="Sample.CrossOriginSupportOptionsHandler, Sample" />
</httpHandlers>