You need a couple of steps here. You would need a cache call back delegate.
This allows you to repopulate the cache when the runtime removes items from
the cache. When this happens, your call back function triggers and you would
then be responsible for repopulating the cache with the item from disk.

You will need to enhance your logic (wrapping the cache object in a class
for instance) so that cache accesses check the cache object first and on a
miss, goes searching for the item on disk. Finding the item on disk
replenishes the cache and returns the info at the same time.

You can either overload the add and insert methods of the cache object or
provide your own wrapper class. It's up to you.

//code provided complements MSDN

static bool itemRemoved = false;
   static CacheItemRemovedReason reason;
   CacheItemRemovedCallback onRemove = null;

   public void RemovedCallback(String k, Object v, CacheItemRemovedReason
r){
     itemRemoved = true;
     reason = r;
   }

   public void AddItemToCache(Object sender, EventArgs e) {
       itemRemoved = false;

       onRemove = new CacheItemRemovedCallback(this.RemovedCallback);

       if (Cache["Key1"] == null)
         Cache.Add("Key1", "Value 1", null, DateTime.Now.AddSeconds(60),
TimeSpan.Zero, CacheItemPriority.High, onRemove);
   }

   public void RemoveItemFromCache(Object sender, EventArgs e) {
       if(Cache["Key1"] != null)
         Cache.Remove("Key1");
   }
//code complements MSDN