C#•3y ago

❔ Filter a ConcurrentDictionary

I have a

ConcurrentDictionary

ConcurrentDictionary

ConcurrentDictionary

ConcurrentDictionary that I'm using for a small in-memory cache and I'm wondering what the best way to filter items out of it. one of the fields on the value stored in the dictionary is a

DateTimeOffset

DateTimeOffset

DateTimeOffset

DateTimeOffset expiration time and I'm still new to using it.

Currently, I'm using

.Select()

.Select()

.Select()

.Select() to filter out the expired items and casting it back to the Dict, overwriting the existing variable with a new one:

_cache = (ConcurrentDictionary<string, UserCacheItem>)_cache.Select(item => item.Value.Expiry > DateTimeOffset.Now);

_cache = (ConcurrentDictionary<string, UserCacheItem>)_cache.Select(item => item.Value.Expiry > DateTimeOffset.Now);

_cache = (ConcurrentDictionary<string, UserCacheItem>)_cache.Select(item => item.Value.Expiry > DateTimeOffset.Now);

_cache = (ConcurrentDictionary<string, UserCacheItem>)_cache.Select(item => item.Value.Expiry > DateTimeOffset.Now);

Is there a better way to do this?

canton7•7/22/23, 6:50 PM

That will throw an exception, because the thing returned by Select isn't a ConcurrentDictionary

canton7•7/22/23, 6:51 PM

Also if you're re-assigning _cache_cache, then you need to be careful about thread-safety. You might need another lock around that field, and consumers might need to take that lock before accessing _cache_cache, etc, and you start to defeat the point of using a concurrent collection in the first place

canton7•7/22/23, 6:52 PM

Life is going to be a lot easier if you make _cache_cache readonlyreadonly, and never assign it. Your approach to removing stale items is then to iterate over it, and remove any values which have expired.

canton7•7/22/23, 6:52 PM

... or you can use a MemoryCacheMemoryCache: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.caching.memorycache?view=dotnet-plat-ext-7.0

13eckOP•7/22/23, 6:53 PM

My initial code did two foreachforeach loops, one to get the expired items and one to remove them, but thought I should do a SelectSelect to cut down on the number of iterations:

List<KeyValuePair<string, UserCacheItem>> Items = new();
foreach (var item in _cache)
{
  if (item.Value.Expiry < DateTimeOffset.Now) Items.Add(item);
}
foreach (var item in Items)
{
  _cache.TryRemove(item);
}

List<KeyValuePair<string, UserCacheItem>> Items = new();
foreach (var item in _cache)
{
  if (item.Value.Expiry < DateTimeOffset.Now) Items.Add(item);
}
foreach (var item in Items)
{
  _cache.TryRemove(item);
}

canton7•7/22/23, 6:55 PM

Yep, that's how I'd do it

13eckOP•7/22/23, 6:57 PM

Thanks for confirming! I recently started C# after being a front-end dev so I'm still learning