WebApi caching passthrough controller - passthrough JSON from another URL, using simple in-memory cache

ContentCache.cs

using System;
using System.Runtime.Caching;
using System.Web.Http;
using System.Xml;

namespace Sixeyed.Api.Core
{
    public class ContentCache
    {
        private static MemoryCache _Cache;
        private static CacheItemPolicy _Policy;
        private static TimeSpan _Lifespan;

        static ContentCache()
        {
            _Cache = new MemoryCache("_ContentCache");
            _Policy = new CacheItemPolicy();
            _Lifespan = XmlConvert.ToTimeSpan("PT10S"); //TODO - from your config
        }

        public static object Get(string id)
        {
            return _Cache[id];
        }

        public static T Get<T>(string id)
        {
            return (T) Get(id);
        }

        public static bool Exists(string id)
        {
            return _Cache[id] != null;
        }

        public static void Set(string id, object content)
        {
            _Policy.AbsoluteExpiration = DateTimeOffset.Now.Add(_Lifespan);
            _Cache.Set(id, content, _Policy);
        }

        public static string ComputeKey<TController>(string id)
            where TController : ApiController
        {
            return ComputeKey(typeof(TController), id);
        }

        public static string ComputeKey(Type type, string id)
        {
            return string.Format("{0}_{1}", type.GetType().Name, id);
        }
    }
}

GitHubController.cs

using System.Web.Http;
using Sixeyed.Api.Core;
using Sixeyed.Api.Core.Controllers;

namespace Sixeyed.Api.Controllers
{
    public class GitHubController : JsonPassthroughController
    {
        //example - caching passthrough for the GitHub status API
        //http://localhost/Sixeyed.Api/api/github/status
        public IHttpActionResult GetStatus()
        {
            return GetFromCacheOrSource("https://status.github.com/api/status.json");
        }
    }
}

JsonPassthroughController.cs

using Newtonsoft.Json.Linq;
using System.Net;
using System.Web.Http;

namespace Sixeyed.Api.Core.Controllers
{
    public abstract class JsonPassthroughController : ApiController
    {
        public IHttpActionResult GetFromCacheOrSource(string sourceUrl)
        {
            var cacheKey = ContentCache.ComputeKey(GetType(), sourceUrl);
            object content = "{}";
            if (!ContentCache.Exists(cacheKey))
            {
                using (var client = new WebClient())
                {
                    try
                    {
                        client.Headers["Accept"] = "application/json";
                        var json = client.DownloadString(sourceUrl);
                        content = JObject.Parse(json);
                        ContentCache.Set(cacheKey, content);
                    }
                    catch (WebException wex)
                    {
                        var response = wex.Response as HttpWebResponse;
                        if (response.StatusCode == HttpStatusCode.NotFound)
                        {
                            return NotFound();
                        }
                        return InternalServerError().
                    }
                }
            }
            else
            {
                content = ContentCache.Get(cacheKey);
            }
            return Ok(content);
        }
    }
}

This additional reference helped me take your code and bypass using CORS to get a project completed. I'm using the server now to retrieve the json I need from another server (another domain) using your code. Very helpful! Thank you for sharing!