possible memory leak in a singleton?

Question

I've asked this question before with no real answer. Can anybody help? I'm profiling the below code inside a singleton and found that a lot of Rate objects (List<Rate>) are kept in memory although I clear them.

protected void FetchingRates()
{
  int count = 0;

  while (true)
  {
    try
    {
      if (m_RatesQueue.Count > 0)
      {
        List<RateLog> temp = null;

        lock (m_RatesQueue)
        {
          temp = new List<RateLog>();
          temp.AddRange(m_RatesQueue);
          m_RatesQueue.Clear();
        }

        foreach (RateLog item in temp)
        {
          m_ConnectionDataAccess.InsertRateLog(item);
        }

        temp.Clear();
        temp = null;
      }
      count++;
      Thread.Sleep(int.Parse(ConfigurationManager.AppSettings["RatesIntreval"].ToString()));
    }
    catch (Exception ex)
    {  
      Logger.Log(ex);                 
    }
  }
} 

The insertion to the queue is made by:

public void InsertLogRecord(RateLog msg)
{
  try
  {
    if (m_RatesQueue != null)
    {
      //lock (((ICollection)m_queue).SyncRoot)
      lock (m_RatesQueue)
      {
        //insert new job to the line and release the thread to continue working.
        m_RatesQueue.Add(msg);
      }
    }
  }
  catch (Exception ex)
  {
    Logger.Log(ex);  
  }
}

The worker inserts rate log into DB as follows:

 internal int InsertRateLog(RateLog item)
    {
        try
        {
            SqlCommand dbc = GetStoredProcCommand("InsertRateMonitoring");
            if (dbc == null)
                return 0;
            dbc.Parameters.Add(new SqlParameter("@HostName", item.HostName));
            dbc.Parameters.Add(new SqlParameter("@RateType", item.RateType));
            dbc.Parameters.Add(new SqlParameter("@LastUpdated", item.LastUpdated));
            return ExecuteNonQuery(dbc);
        }
        catch (Exception ex)
        {
            Logger.Log(ex);
            return 0;
        }
    }

Any one sees a possible memory leak?

Answer 1

Looks like you are not disposing of your SqlCommand which is hanging onto a RateLog.

Answer 2

1. I hope you are properly disposing the ADO.NET objects. (This is simply good practice.)
2. Any stray references will keep your RateLog objects from being collected by the GC.

I recommend you look through your code starting where the RateLog objects are being created and take note of all the places a reference is being held. Here are some things to consider.

1. Are the RateLog objects subscribed to any events?
2. Are you keeping a collection of RateLog objects sitting somewhere in a static class?

You should also consider encapsulating all your thread safety boilerplate in class.

public sealed class WorkQueue<T>
{
    private readonly System.Collections.Generic.Queue<T> _queue = new System.Collections.Generic.Queue<T>();
    private readonly object _lock = new object();

    public void Put(T item)
    {
        lock (_lock)
        {
            _queue.Enqueue(item);
        }
    }


    public bool TryGet(out T[] items)
    {
        if (_queue.Count > 0)
        {
            lock (_lock)
            {
                if (_queue.Count > 0)
                {
                    items = _queue.ToArray();
                    _queue.Clear();
                    return true;
                }
            }
        }

        items = null;
        return false;
    }
}

This will make your code a lot clearer:

protected void FetchingRates()
{
    int ratesInterval = int.Parse(ConfigurationManager.AppSettings["RatesIntreval"].ToString());
    int count = 0;
    var queue = new WorkQueue<RateLog>();

    while (true)
    {
        try
        {
            var items = default(RateLog[]);
            if (queue.TryGet(out items))
            {
                foreach (var item in items)
                {
                    m_ConnectionDataAccess.InsertRateLog(item);
                }
            }
        }
        catch (Exception ex)
        {  
            Logger.Log(ex);                 
        }

        Thread.Sleep(ratesInterval);
        count++;
    }
} 

Answer 3

the Clear() function deconstructs the List. But what about the RateLog instances? Is their deconstructor called? What about the lock, maybe this prevents that the RateLog from beeing deleted.

Answer 4

How about moving the temp creation outside the loop. You are probably not allowing the GC to clean up.

protected void FetchingRates()
{
  int count = 0;
  List<RateLog> temp = new List<RateLog>();

  while (true)
  {
    try
    {
      if (m_RatesQueue.Count > 0)
      {    
        lock (m_RatesQueue)
        {
          temp.AddRange(m_RatesQueue);
          m_RatesQueue.Clear();
        }

        foreach (RateLog item in temp)
        {
          m_ConnectionDataAccess.InsertRateLog(item);
        }

        temp.Clear();
      }
      count++;
      Thread.Sleep(int.Parse(ConfigurationManager.AppSettings["RatesIntreval"].ToString()));
    }
    catch (Exception ex)
    {                   
    }
  }
} 

After temp.Clear() you might try adding GC.Collect();. This should NOT be the permanent solution, but could be used for your profiling to see if the objects get cleaned up eventually. If not, then there might still be a reference or event attached somewhere.