If you have two projects , like a Models or Entities project and your web project all you need to do is add a reference in the Models project to System.Web. If you did not add a reference to System.Web then you won't have access to HttpRuntime or HttpContect even though there is a namespace for System.Web.

After you do so, you'll have access to:

System.Web.HttpRuntime.Cache

and could use

HttpRuntime.Cache.Insert(
                            CacheKey,
                            CacheValue,
                            null,
                            DateTime.Now.AddMinutes(CacheDuration),
                            Cache.NoSlidingExpiration
                        );
                        Hashtable table1 = HttpRuntime.Cache[CacheKey] as Hashtable;
                        HttpRuntime.Cache.Remove(CacheKey);

You'll also have access to

System.Web.HttpContext.Current.Application

and could use

System.Web.HttpContext.Current.Application.Add("table1",myHashTable);
Hashtable table1 = System.Web.HttpContext.Current.Application["table1"] as Hashtable;