包含关键线程安全 [英] ContainsKey Thread Safe

问题描述

在以下代码中：

public class StringCache
{
    private readonly object lockobj = new object();

    private readonly Dictionary<int, string> cache = new Dictionary<int, string>();

    public string GetMemberInfo(int key)
    {
        if (cache.ContainsKey(key))
            return cache[key];
        lock (lockobj)
        {
            if (!cache.ContainsKey(key))
                cache[key] = GetString(key);
        }
        return cache[key];
    }

    private static string GetString(int key)
    {
        return "Not Important";
    }
}

1）ContainsKey线程是否安全？ IOW，如果在另一个线程向字典添加内容时执行该方法，会发生什么？
2）对于第一个返回缓存[key]，有没有机会返回乱码？

1) Is ContainsKey thread safe? IOW, what happens if that method is executing when another thread is adding something to the dictionary? 2) For the first return cache[key], is there any chance that it could return a garbled value?

TIA，

MB

推荐答案

ContainsKey的固有线程安全性并不重要，因为ContainsKey&缓存[key]。

The inherent thread safety of ContainsKey doesn't matter, since there is no synchronization between ContainsKey & cache[key].

例如：

if (cache.ContainsKey(key))
   // Switch to another thread, which deletes the key.
   return cache[key];

MSDN在这一点上非常明确：

MSDN is pretty clear on this point:

为了允许通过多个线程读取
来收集集合，您必须实现自己的
同步。

To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

有关更多信息，JaredPar在 http://blogs.msdn.com/jaredpar/archive/2009/02/11/why-are-thread- safe-collections-so-hard.aspx 在线程安全的集合。

For more info, JaredPar posted a great blog entry at http://blogs.msdn.com/jaredpar/archive/2009/02/11/why-are-thread-safe-collections-so-hard.aspx on thread-safe collections.

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