C# Memory leak with ConcurrentQueue

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i have memory leak when using ConcurrentQueue :

requestObject request = xxx;

Item obj= new Item ();
obj.MessageReceived += obj_MessageReceived;
obj.Exited += obj_Exited;

request.Key = obj.Key;

obj.AddRequest(request);

_queue.TryAdd(obj.Key, obj);

In the "Exited" callback, i dispose the resource :

void LiveSphere_Exited(string key)
{
   Item instance;

   _queue.TryRemove(key, out instance);

Task.Factory.StartNew(() =>
{
    var wait = new SpinWait();
                while (instance.MessageCount > 0)
                {
                    wait.SpinOnce();
                }
            }).ContinueWith((t) =>
            {
                if (instance != null)
                {
                    //Cleanup resources
                    instance.MessageReceived -= obj_MessageReceived;
                    instance.Exited -= obj_Exited;
                    instance.Dispose();
                    instance = null;
                }
            });

When i profile the code, i still have a root referenced "Item" object but i don't know where i can dispose..., The exited method is triggered and the _queue has removed the "Item" object from the queue.

When i read documentation, the concurrentqueue copy the reference into the queue.

Can you help me to find out where the memory leak is?

Thank you,

Kind Regards.