Cache cache = System.Web.HttpRuntime.Cache;

var config = new NameValueCollection();
var cache = new MemoryCache("myMemCache", config);
cache.Add(new CacheItem("a", "b"),
    new CacheItemPolicy
    {
        Priority = CacheItemPriority.NotRemovable,
        SlidingExpiration=TimeSpan.FromMinutes(30)
    });