Fix the symbol cache and image cache

PortfolioManager/Cache/ImageCache.cs

@@ -57,6 +57,7 @@ namespace PortfolioManager.Cache
     {
       lock (thisLock)
       {
+        DisposeBitmaps();
         imageCache = new Dictionary<ImageType, Bitmap>();
       }
     }
@@ -66,21 +67,29 @@ namespace PortfolioManager.Cache
       lock (thisLock)
       {
         if (null == imageCacheInstance) return;
-        List<Bitmap> bitmaps = imageCache.Values.ToList();
-        foreach (Bitmap bitmap in bitmaps)
-        {
-          bitmap.Dispose();
-        }
+        DisposeBitmaps();
         imageCache = null;
         imageCacheInstance = null;
       }
     }
     
-    public IImage GetImage(ImageCache.ImageType imageType)
+    private void DisposeBitmaps()
     {
-      lock(this)
+      if(null == imageCache)return;
+      List<Bitmap> bitmaps = imageCache.Values.ToList();
+      foreach (Bitmap bitmap in bitmaps)
       {
-        return imageCache[imageType];
+        bitmap.Dispose();
+      }
+    }
+
+    public IImage GetImage(ImageType imageType)
+    {
+        lock(thisLock)
+        {
+          if (imageCache == null)throw new ObjectDisposedException(nameof(ImageCache));
+          if (!imageCache.TryGetValue(imageType, out var bitmap))throw new KeyNotFoundException($"Image {imageType} not found in cache.");
+          return bitmap;
         }
     }    
   }

PortfolioManager/Cache/SymbolCache.cs

@@ -10,7 +10,8 @@ namespace PortfolioManager.Cache
   public class SymbolCache : IDisposable
   {
     private List<String> symbolCache=new List<String>();
-    private Object thisLock=new Object();
+    private readonly Object thisLock=new Object();
+    private readonly Object fetchLock = new Object();
     private Thread cacheMonitorThread=null;
     private volatile bool threadRun=true;
     private int cacheRefreshAfter=60000;        // Invalidate cache after
@@ -60,6 +61,8 @@ namespace PortfolioManager.Cache
       {
         if(symbolCache.Count>0)return new List<string>(symbolCache);
       }
+      lock(fetchLock)
+      {
         List<String> symbols = PricingDA.GetSymbols();
         lock(thisLock)
         {
@@ -68,6 +71,7 @@ namespace PortfolioManager.Cache
           return new List<string>(symbols);
         }
       }
+    }
 
     private void ThreadProc()
     {