Question

I need some directions here.
I have the following key/value cache:

public class Cache<TKey, TValue> : ICache<TKey, TValue>
{
    private readonly IDictionary<TKey, TValue> _internalCache;
    private readonly object _syncLock = new object();

    public Cache()
    {
        _internalCache = new Dictionary<TKey, TValue>();
    }

    public TValue this[TKey key]
    {
        get
        {
            lock (_syncLock) {
               //...
            }
        }
        set
        {
            lock (_syncLock) {
               //...
            }
        }
    }

    public ICollection<TValue> GetAll()
    {
        lock (_syncLock) {
            return _internalCache.Values;
        }
    }

    public bool ContainsKey(TKey key)
    {
        lock (_syncLock)
        {
            return _internalCache.ContainsKey(key);
        }
    }

}   

The cache above is used by a singleton wrapper:

 public class ActivityCache : ICache<string, Activity> 
 {
    private readonly ICache<string, Activity> _cache = new Cache<string, Activity>();

    private static readonly ActivityCache _instance = new ActivityCache();

    // http://www.yoda.arachsys.com/csharp/singleton.html
    static ActivityCache()
    {
    }

    ActivityCache()
    {
    }

    public static ActivityCache Instance
    {
        get { return _instance; }
    }

    public Activity this[string activityUrl]
    {
        get
        {
            if (string.IsNullOrEmpty(activityUrl))
            {
                return null;
            }

            return _cache[activityUrl];
        }
        set
        {
            if (string.IsNullOrEmpty(activityUrl))
            {
                return;
            }

            _cache[activityUrl] = value;
        }
    }

    public ICollection<Activity> GetAll()
    {
        return _cache.GetAll();
    }

    public bool ContainsKey(string key)
    {
        return _cache.ContainsKey(key);
    }
}

This is working fine (I haven’t noticed/heard of any errors… yet 🙂 ).

But now I have a problem. I need to reload the cache with new key/values.

Question 1.) Can I implement a “safe” reload method that reloads the cache (the Dictionary in the Cache class) ?

E.g:

    public void Reload(IDictionary<TKey, TValue> values)
    {
        lock (_syncLock)
        {
            _internalCache.Clear();
            foreach (KeyValuePair<TKey, TValue> value in values)
            {

                /* Problems can (will) occur if another 
                   thread is calling the GetAll method... */
                _internalCache[value.Key] = value.Value;
            }
        }
    }

Question 2.) Should I use some IoC container or some other library instead?

Thanks!

Note: I’m using .NET 3.5

Answer 1

You should lock around GetAll as well.

You could use a double buffer type technique to make the reload less painful :

public void Reload(IDictionary<TKey, TValue> values)
{
    cache = new Dictionary<TKey, TValue> ();
    foreach (KeyValuePair<TKey, TValue> value in values)
    {
        cache[value.Key] = value.Value;
    }

    lock (_syncLock)
    {
        _internalCache = cache;
    }
}

This will work providing you don’t mind readers accessing potentially out of date information whilst you call reload.