一个优雅的Lock方案

虽然复杂，但是考虑到业务逻辑的需要，这套Lock方案还是相当不错的：

   1: CacheItem cacheItemBeforeLock = null;

   2: bool lockWasSuccessful = false;

   3:  

   4: do

   5: {

   6:     lock (inMemoryCache.SyncRoot)

   7:     {

   8:         if (inMemoryCache.Contains(key) == false)

   9:         {

  10:             cacheItemBeforeLock = new CacheItem(key, addInProgressFlag, CacheItemPriority.NotRemovable, null);

  11:             inMemoryCache[key] = cacheItemBeforeLock;

  12:         }

  13:         else

  14:         {

  15:             cacheItemBeforeLock = (CacheItem)inMemoryCache[key];

  16:         }

  17:  

  18:         lockWasSuccessful = Monitor.TryEnter(cacheItemBeforeLock);

  19:     }

  20:  

  21:     if (lockWasSuccessful == false)

  22:     {

  23:         Thread.Sleep(0);

  24:     }

  25: } while (lockWasSuccessful == false);

  26:  

  27: try

  28: {

  29:     cacheItemBeforeLock.TouchedByUserAction(true);

  30:  

  31:     CacheItem newCacheItem = new CacheItem(key, value, scavengingPriority, refreshAction, expirations);

  32:     try

  33:     {

  34:         backingStore.Add(newCacheItem);

  35:         cacheItemBeforeLock.Replace(value, refreshAction, scavengingPriority, expirations);

  36:         inMemoryCache[key] = cacheItemBeforeLock;

  37:     }

  38:     catch

  39:     {

  40:         backingStore.Remove(key);

  41:         inMemoryCache.Remove(key);

  42:         throw;

  43:     }

  44:  

  45:     if (scavengingPolicy.IsScavengingNeeded(inMemoryCache.Count))

  46:     {

  47:         cacheScavenger.StartScavenging();

  48:     }

  49:  

  50:     instrumentationProvider.FireCacheUpdated(1, inMemoryCache.Count);

  51: }

  52: finally

  53: {

  54:     Monitor.Exit(cacheItemBeforeLock);

  55: }