How to refresh singleton in C#

Question

I have singleton that fetches from DB, hence it is expensive load. It is lazy loaded.

I would like to create a method that refreshes that singleton and populates it once it is required.

the data is DB and very expensive, so I want to refresh it only once in case I have concurrent calls. (that is, if I get 500 calls to refresh, I want to restart the refresh only once)

public static PageData Instance
    {
        get
        {
            if (m_Instance == null)
            {
                lock (instanceLock)
                {
                    if (m_Instance == null)
                    {
                        m_Instance = new PageData();
                    }
                }
            }
            return m_Instance;
        }
    }


public void ReSync()
        {                         
            lock (instanceLock)
            {
                /* Setting to null to force the Instance to re-build */
                m_Instance = null;
                PageData pData = Instance;
            }
        }

thanks

Answer 1

This is slightly incorrect, your

if (m_Instance == null)

should really be on the inside of the lock.

Sorry, didn't spot that.

Nothing is built in that can make other clients abandon calls silently if you are already refreshing. Timeouts will generate an exception I think. Perhaps maintain a stale DateTime that you can check to avoid doing the refresh on queued callers.

Answer 2

In my understanding this should work.

Here is my code:

private static instanceLock = new object();
private static _refreshing = true;

public static PageData Instance
    {
        get
        {
            if (_refreshing)
            {
                lock (instanceLock)
                {
                    if (_refreshing)
                    {
                        m_Instance = new PageData();
                        _refreshing = false; //now allow next refreshes.
                    }
                }
            }
            return m_Instance;
        }
    }


public void ReSync()
        {
            if (!_refreshing)                         
                lock (instanceLock)
                {
                    if (!_refreshing)
                    {
                        _refreshing = true; //don't allow refresh until singleton is called.
                    }
                }
        }

Answer 3

Aside from the double-checked locking being broken (correction, apparently it does work but I still find it not particularly pretty), if you have write access to m_Instance why not just set it to new PageData() there and then in ReSync?