这是ConcurrentDictionary和ConcurrentQueue线程安全的组合吗？

Question

我在以下代码中使用.NET 4中的ConcurrentDictionary和ConcurrentQueue类.

这段代码是线程安全的吗？如果没有,我怎样才能使它成为线程安全的？

public class Page
{
    public string Name {get; set; }
}

public class PageQueue
{
    private ConcurrentDictionary<int, ConcurrentQueue<Page>> pages =
        new ConcurrentDictionary<int, ConcurrentQueue<Page>>();

    public void Add(int id, Page page)
    {
        if (!this.pages.ContainsKey(id))
            this.pages[id] = new ConcurrentQueue<Page>();

        this.pages[id].Enqueue(page);
    }

    public Page GetAndRemove(int id)
    {
        Page lp = null;

        if(this.pages.ContainsKey(id))
            this.pages[id].TryDequeue(out lp);

        return lp;
    }
}

演示:

 public class Demo
 {
     public void RunAll()
     {
         for (int i = 0; i < 10; i++)
             Task.Factory.StartNew(() => Run());
     }

     public void Run()
     {
         PageQueue pq = new PageQueue();
         pq.Add(1, new Page());

         pq.GetAndRemove(1);
     }
 }

Answer 1

正如@Femaref正确指出的那样,代码中存在一些缺陷.我建议你利用ConcurrentDictionary <K,V>提供的许多方法,使代码在线程安全,而不需要lock语句:

public class PageQueue
{
    private ConcurrentDictionary<int, ConcurrentQueue<Page>> pages =
        new ConcurrentDictionary<int, ConcurrentQueue<Page>>();

    public void Enqueue(int id, Page page)
    {
        var queue = this.pages.GetOrAdd(id, _ => new ConcurrentQueue<Page>());

        queue.Enqueue(page);
    }

    public bool TryDequeue(int id, out Page page)
    {
        ConcurrentQueue<Page> queue;

        if (this.pages.TryGetValue(id, out queue))
        {
            return queue.TryDequeue(out page);
        }

        page = null;
        return false;
    }
}