getting all values from a concurrent dictionary and clearing it without losing data

Question

I am adding/updating objects into a concurrent dictionary and periodically (every minute) flushing the dictionary, so my code looks something like this:

    private static ConcurrentDictionary<string, Metric> _metrics = new ConcurrentDictionary<string, Metric>();

    public static void IncrementCountMetricBy(string name, int count)
    {           
        _metrics.AddOrUpdate(....
    }

    public static Metric[] Flush()
    {
        var flushedMetrics = _metrics;
        _metrics = new ConcurrentDictionary<string, Metric>();
        return flushedMetrics.Values.ToArray();
    }

now I'm not sure if it's possible for this code to lose some objects/updates

Answer 1

Yes, you could potentially lose some data there:

1. The incrementing thread could read the _metrics field and get the old dictionary, and then be interrupted
2. The flushing thread then replaces the _metrics field with the new dictionary
3. The flushing thread than called Values.ToArray()
4. The incrementing thread then calls AddOrUpdate on a dictionary which is no longer being looked at by anything. (The one that it fetched in step 1.)

To put it another way, imagine that your IncrementMetricCountBy method is actually:

public static void IncrementCountMetricBy(string name, int count)
{
    var tmp = _metrics;
    Thread.Sleep(1000);           
    tmp.AddOrUpdate(...);
}

If you can see why that isn't safe, the same argument applies in your current code.

As far as I can see there isn't anything particularly simple that you can do with ConcurrentDictionary here. One option would be to take a snapshot of all the keys, and then remove them all:

var keys = _metrics.Keys.ToList();
var values = new List<Metric>();
foreach (var key in keys)
{
    Metric metric;
    if (_metrics.TryRemove(key, out metric))
    {
        values.Add(metric);
    }
}
return values;

The dictionary may not be empty when you return, but you shouldn't lose any data. (You may get metrics updated since the method started, and any update which happens after a key has been removed will end up re-adding it, but that should be okay.)