Generally, one remembers to lock() or Monitor.TryEnter() their resources, in a multi-threaded situation. Collections are often needed by multiple threads, and can be made static, to make them generally accessible to all threads. Furthermore, collections support concurrent readers, making them ideal for multi-thread access.
So, what happens if another thread writes to the static dictionary, while reading? Well, it isn't a disaster, nor a deadlock, but it sure gets slow! The problem is the reader(s) contend with the writer, as the dictionary keys are being updated. If several threads are attempting to traverse the keys, performance suffers greatly, and you'll eventually receive an InvalidOperationException.
In the threading section of the Dictionary<T> MSDN documentation, we find:
"Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
A Dictionary<TKey, TValue> can support multiple readers concurrently... blah blah blah"Many programmers (unfortunately, myself included) stop reading at that point, and return to coding with a smile, because they discovered a thread safe, concurrent read collection. If we keep on reading, we find the answer to the problem:
A Dictionary<TKey, TValue> can support multiple readers concurrently, ...as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.Simply locking the resource prevents the whole issue. It is well worth the time to insert Monitor.TryEnter statements throughout the code, rather than suffer intermittent performance issues.
*Check out the Tess Ferrandez's blog post, High CPU in .NET app using a static Generic.Dictionary. Her post is directly related to this one, and provides debug details.