const int MAX_SIZE = 1000000;
static BlockingCollection<string> mergeData;
mergeData = new BlockingCollection<string>(new ConcurrentBag<string>(), MAX_SIZE);


string[] FilePaths = Directory.GetFiles("somepath");

var taskWriteMergedFile = new Task(() =>
{
    while ( true )
    {
        if ( mergeData.Count  > 800000)
        {
            String.Join(System.Environment.NewLine, mergeData.GetConsumingEnumerable());
            //Write to file
        }
        Thread.Sleep(10000);
    }
}, TaskCreationOptions.LongRunning);

taskWriteMergedFile.Start();
Parallel.ForEach(FilePaths, FilePath => AddToDataPool(FilePath));
mergeData.CompleteAdding();