Question

For example can I go:

string[] keys = new string[items.Count];
items.Keys.CopyTo(keys);

Where items is a:

Dictionary<string, MyObject>

instance that could be being modified by another thread? (Its not clear to me if when accessing the Keys property the inner workings of Dictionary iterates the collection – in which case I know it would not be thread safe).

Update: I realise my above example is not thread safe becuase the size of the Dictonary could change between the lines. however what about:

Dictionary<string, MyObject> keys = items.Keys;
string[] copyOfKeys = new string[keys.Count];
keys.Keys.CopyTo(copyOfKeys );

Answer 1

No, Dictionary is not thread-safe. If you need thread safety and you’re using .NET 4 or higher, you should use a ConcurrentDictionary.

From MSDN:

A Dictionary 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.