Question

In the following code:

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) 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

Answer 1

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

For example:

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

MSDN is pretty clear on this point:

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

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.