Fix symbol cache

PortfolioManager/Cache/SymbolCache.cs

@@ -36,12 +36,15 @@ namespace PortfolioManager.Cache
         symbolCache=new List<string>();
       }
     }
+
     public void Dispose()
     {
       lock(thisLock)
       {
 			  if(null==symbolCacheInstance || false==threadRun)return;
         threadRun=false;
+        symbolCacheInstance=null;
+      }
       if(null!=cacheMonitorThread)
       {
         MDTrace.WriteLine(LogLevel.DEBUG,String.Format("[SymbolCache:Dispose]Thread state is {0}.  Joining main thread...",Utility.ThreadStateToString(cacheMonitorThread)));
@@ -49,18 +52,23 @@ namespace PortfolioManager.Cache
         cacheMonitorThread=null;
         MDTrace.WriteLine(LogLevel.DEBUG,"[SymbolCache:Dispose] End.");
       }
-        symbolCacheInstance=null;
-      }
     }
 
     public List<String> GetSymbols()
     {
-      lock(this)
+      lock(thisLock)
       {
-        if(0==symbolCache.Count)symbolCache=PricingDA.GetSymbols();
-        return symbolCache;
+        if(symbolCache.Count>0)return new List<string>(symbolCache);
+      }
+      List<String> symbols = PricingDA.GetSymbols();
+      lock(thisLock)
+      {
+        if(symbolCache.Count>0)return new List<string>(symbolCache);
+        symbolCache=new List<String>(symbols);
+        return new List<string>(symbols);
       }
     }
+
     private void ThreadProc()
     {
       int quantums=0;
@@ -68,13 +76,15 @@ namespace PortfolioManager.Cache
       while(threadRun)
       {
         Thread.Sleep(quantumInterval);
+        if(!threadRun)break;
         quantums+=quantumInterval;
         if(quantums>cacheRefreshAfter)
         {
           quantums=0;
+          List<String> symbols =  PricingDA.GetSymbols();
           lock(thisLock)
           {
-            symbolCache=PricingDA.GetSymbols();
+            symbolCache=new List<string>(symbols);
           }
         }
       }