Is a linq query to ConcurrentDictionary Values threadsafe?

Question

let\'s say I have the following code:

ConcurrentDictionary myDict= new ConcurrentDictionary();

Normally

Answer 1

All of the answers so far are great and helpful, but I think the link that DuneCat pointed out in one of the comments bears emphasis:

http://geekswithblogs.net/simonc/archive/2012/02/22/inside-the-concurrent-collections-concurrentdictionary.aspx

Specifically.....

Lockless:

• TryGetValue
• GetEnumerator
• The indexer getter
• ContainsKey

Takes out every lock (lockfull?):

• Count
• IsEmpty
• Keys
• Values
• CopyTo
• ToArray

Answer 2

The documentation of the ConcurrentDictionary states (MSDN):

All public and protected members of ConcurrentDictionary are thread-safe and may be used concurrently from multiple threads.

Since the .Values property is an implementation dictated by the IColletion interface it is public and therefore thread safe.

Answer 3

Correction... I'm not sure when you are accessing the Values property. It is thread safe when using LINQ on the object itself.

---

LINQ will use the GetEnumerator method to itterate the items.

Straight from MSDN

The enumerator returned from the dictionary is safe to use concurrently with reads and writes to the dictionary, however it does not represent a moment-in-time snapshot of the dictionary. The contents exposed through the enumerator may contain modifications made to the dictionary after GetEnumerator was called

if myDict.Any(x => !x.Value.HasPaid))
{
  return false
}

Answer 4

As already mentioned, ConcurrentDictionary.GetEnumerator() does not represent a moment-in-time snapshot of the dictionary. However, ConcurrentDictionary.Values does produce a moment-in-time snapshot.

Therefore the following are not equivalent:

myDict.Any(x => !x.Value.HasPaid)
myDict.Values.Any(x => !x.HasPaid)