diff --git a/SmartThreadPool/SmartThreadPool.cs b/SmartThreadPool/SmartThreadPool.cs
index dc067a5..921cfb1 100644
--- a/SmartThreadPool/SmartThreadPool.cs
+++ b/SmartThreadPool/SmartThreadPool.cs
@@ -902,7 +902,7 @@ internal static void ValidateWorkItemsGroupWaitForIdle(IWorkItemsGroup workItems
             ValidateWorkItemsGroupWaitForIdleImpl(workItemsGroup, workItem);
 			if ((null != workItemsGroup) &&
                 (null != workItem) &&
-                CurrentThreadEntry.CurrentWorkItem.WasQueuedBy(workItemsGroup))
+                workItem.WasQueuedBy(workItemsGroup))
 			{
 				throw new NotSupportedException("WaitForIdle cannot be called from a thread on its SmartThreadPool, it causes a deadlock");
 			}
@@ -1474,6 +1474,11 @@ public static bool IsWorkItemCanceled
         {
             get
             {
+                if (CurrentThreadEntry == null || CurrentThreadEntry.CurrentWorkItem == null)
+                {
+                    throw new InvalidOperationException(
+                        "IsWorkItemCanceled can only be called from a work item executing in the SmartThreadPool");
+                }
                 return CurrentThreadEntry.CurrentWorkItem.IsCanceled;
             }
         } 
diff --git a/SmartThreadPool/SynchronizedDictionary.cs b/SmartThreadPool/SynchronizedDictionary.cs
index 9572a58..d0e472d 100644
--- a/SmartThreadPool/SynchronizedDictionary.cs
+++ b/SmartThreadPool/SynchronizedDictionary.cs
@@ -16,7 +16,13 @@ public SynchronizedDictionary()
 
         public int Count
         {
-            get { return _dictionary.Count; }
+            get
+            {
+                lock (_lock)
+                {
+                    return _dictionary.Count;
+                }
+            }
         }
 
         public bool Contains(TKey key)
diff --git a/SmartThreadPool/WorkItemsGroup.cs b/SmartThreadPool/WorkItemsGroup.cs
index 14740fd..0a1ee0b 100644
--- a/SmartThreadPool/WorkItemsGroup.cs
+++ b/SmartThreadPool/WorkItemsGroup.cs
@@ -115,11 +115,14 @@ public override int Concurrency
             {
                 Debug.Assert(value > 0);
 
-                int diff = value - _concurrency;
-                _concurrency = value;
-                if (diff > 0)
+                lock (_lock)
                 {
-                    EnqueueToSTPNextNWorkItem(diff);
+                    int diff = value - _concurrency;
+                    _concurrency = value;
+                    if (diff > 0)
+                    {
+                        EnqueueToSTPNextNWorkItem(diff);
+                    }
                 }
             }
         }
@@ -165,14 +168,17 @@ public override WIGStartInfo WIGStartInfo
 	    /// </summary>
 	    public override void Start()
 	    {
-	        // If the Work Items Group already started then quit
-	        if (!_isSuspended)
+	        lock (_lock)
 	        {
-	            return;
+	            // If the Work Items Group already started then quit
+	            if (!_isSuspended)
+	            {
+	                return;
+	            }
+	            _isSuspended = false;
+
+	            EnqueueToSTPNextNWorkItem(Math.Min(_workItemsQueue.Count, _concurrency));
 	        }
-	        _isSuspended = false;
-            
-	        EnqueueToSTPNextNWorkItem(Math.Min(_workItemsQueue.Count, _concurrency));
 	    }
 
 	    public override void Cancel(bool abortExecution)
@@ -219,12 +225,15 @@ private void RegisterToWorkItemCompletion(IWorkItemResult wir)
 
 	    public void OnSTPIsStarting()
 		{
-            if (_isSuspended)
+            lock (_lock)
             {
-                return;
+                if (_isSuspended)
+                {
+                    return;
+                }
+
+                EnqueueToSTPNextNWorkItem(_concurrency);
             }
-			
-            EnqueueToSTPNextNWorkItem(_concurrency);
 		}
 
 	    public void EnqueueToSTPNextNWorkItem(int count)
diff --git a/SmartThreadPool/WorkItemsQueue.cs b/SmartThreadPool/WorkItemsQueue.cs
index 8eaf593..fb727b0 100644
--- a/SmartThreadPool/WorkItemsQueue.cs
+++ b/SmartThreadPool/WorkItemsQueue.cs
@@ -582,11 +582,14 @@ public void Dispose()
 
         public void Dispose()
         {
-            if (!_isDisposed)
+            lock (this)
             {
-                Cleanup();
+                if (!_isDisposed)
+                {
+                    Cleanup();
+                    _isDisposed = true;
+                }
             }
-            _isDisposed = true;
         }
 
         private void ValidateNotDisposed()