我想让我的代码多线程化，因此我需要把字典变成ConcurrentDictionary。我读了关于ConcurrentDictionary的文章，检查了一些例子，但我仍然需要一个帮手：
下面是原始代码（对于单线程）

private IDictionary<string, IDictionary<string, Task>> _tasks;

public override IDictionary<string, IDictionary<string, Task>> Tasks
{
    get
    {
        // return dictionary from cache unless too old
        // concurrency!! (null check)
        if (_tasks != null && (DateTime.Now - _lastTaskListRefreshDateTime < TimeSpan.FromSeconds(30)))
        {
            return _tasks;
        }

        // reload dictionary from database
        _tasks = new Dictionary<string, IDictionary<string, Task>>();

        // find returns an IEnumerable<Task>
        var tasks = Find<Task>(null, DependencyNode.TaskForCrawler).Cast<Task>();

        // build hierarchical dictionary from flat IEnumerable
        // concurrency!!
        foreach (var t in tasks)
        {

            if (_tasks.ContainsKey(t.Area.Key))
            {
                if (_tasks[t.Area.Key] == null)
                {
                    _tasks[t.Area.Key] = new Dictionary<string, Task>();
                }

                if (!_tasks[t.Area.Key].ContainsKey(t.Key))
                {
                    _tasks[t.Area.Key].Add(t.Key, t);
                }
            }
            else
            {
                _tasks.Add(t.Area.Key, new Dictionary<string, Task> { { t.Key, t } });
            }
        }

        _lastTaskListRefreshDateTime = DateTime.Now;
        return _tasks;
    }

    set
    {
        _tasks = value;
    }
}

以下是我的结论：

private ConcurrentDictionary<string, ConcurrentDictionary<string, Task>> _tasks = new ConcurrentDictionary<string, ConcurrentDictionary<string, Task>>();

public override ConcurrentDictionary<string, ConcurrentDictionary<string, Task>> Tasks
{
    get
    {
        // use cache
        // concurrency?? (null check)
        if (!_tasks.IsEmpty && (DateTime.Now - _lastTaskListRefreshDateTime < TimeSpan.FromSeconds(30)))
        {
            return _tasks;
        }

        // reload
        var tasks = Find<Task>(null, DependencyNode.TaskForCrawler).Cast<Task>();

        foreach (var task in tasks)
        {
            var t = task; // inner scope for clousure
            var taskKey = t.Key;
            var areaKey = t.Area.Key;

            var newDict = new ConcurrentDictionary<string, Task>();
            newDict.TryAdd(taskKey, t);

            _tasks.AddOrUpdate(areaKey, newDict, (k, v) => {
                                                    // An dictionary element if key=areaKey already exists
                                                    // extend and return it.
                                                    v.TryAdd(taskKey, t);
                                                    return v;
                                                   });
        }

        _lastTaskListRefreshDateTime = DateTime.Now;
        return _tasks;
    }
}

我不确定这是不是，特别是我很确定IsEmpty检查不是线程安全的，因为_tasks可能在IsEmpty检查和&& ...部分或return _tasks部分之间初始化了。我必须手动锁定这个检查吗？我需要双重锁定吗（null check〉lock〉null check）？