Question

If I have a ConcurrentDictionary and use the TryGetValue within an if statement, does this make the if statement’s contents thread safe? Or must you lock still within the if statement?

Example:

        ConcurrentDictionary<Guid, Client> m_Clients;

        Client client;
        //Does this if make the contents within it thread-safe? 
        if (m_Clients.TryGetValue(clientGUID, out client))
        {
            //Users is a list.
            client.Users.Add(item);
        }

or do I have to do:

        ConcurrentDictionary<Guid, Client> m_Clients;

        Client client;
        //Does this if make the contents within it thread-safe? 
        if (m_Clients.TryGetValue(clientGUID, out client))
        {
            lock (client)
            {
                //Users is a list.
                client.Users.Add(item);
            }
        }

Answer 1

Yes you have to lock inside the if statement the only guarantee you get from concurrent dictionary is that its methods are thread save.