.Net 4.0 MemoryCache Clearing - wcf

Question

I am using a .Net 4.0 MemoryCache in my WCF service.
I originally was using the Default Cache as below:
var cache = MemoryCache.Default;
Then doing the usual pattern as trying to find something in the Cache, getting, then
setting into the cache if did not find (code snippet / pseudo code as below):
var geoCoordinate = cache.Get(cacheKey) as GeoCoordinate;
if (geoCoordinate == null)
{
geoCoordinate = get it from somewhere;
cache.Set(cacheKey, geoCoordinate, DateTimeOffset.Now.AddDays(7));
}
I was finding that my entries were disappearing after approx. 2 minutes. Even if my code placed the missing entries back into the cache, subsequent cache Gets would return null.
My WCF Service is being hosted by IIS 7.5. If I recycled the App Pool, everything would work normally for 2 minutes, but then the pattern as described above would repeat.
After doing some researching I then did the below to replace:
var cache = MemoryCache.Default;
// WITH NEW CODE TO CREATE CACHE AS BELOW:
var config = new NameValueCollection();
//Hack: Set Polling interval to 10 days, so will no clear cache.
config.Add("pollingInterval", "10:00:00:00");
config.Add("physicalMemoryLimitPercentage", "90");
config.Add("cacheMemoryLimitMegabytes", "2000");
//instantiate cache
var cache = new MemoryCache("GeneralCache", config);
It seems that no matter what I place into physicalMemoryLimitPercentage, or cacheMemoryLimitMegabytes does not seem to help. But placing the pollingInterval to a large datespan does.
ie: If I set as below:
config.Add("pollingInterval", "00:00:15:00");
Then everything works fine for 15 minutes.
Note: If my WCF service is hosted by IISExpress on my dev environment, I cannot reproduce.
This also seems to happen when my WCF service is hosted by IIS 7.5.
My app pool on IIS 7.5 is NOT recycling.
Has anybody experienced something like this?
I have seen the below:
MemoryCache does not obey memory limits in configuration
Thanks,
Matt

Answer 1

I too have seen this issue and filed a bug with MS here with a simple reproducer project.
This has been resolved by MS in the above bug I filed - with a work around there and an upcoming QFE for .net 4 as well as confirmation that this isn't a problem in 4.5
I have not yet tried the work around
I can however give some more information on conditions required by myself to recreate this. The application pool needed to be in Integrated Pipeline mode for me to see this issue - Classic mode fixes this issue though removes some of the benefits of moving to IIS 7.5.
Equally when using Integrated mode I also did not see this issue if I used a built-in application pool identity such as ApplicationPoolIdentity. However my app needs to run as a custom identity using a service account and it is at this point at which I see the behavior. Therefore if you don't need Integrated mode or a custom Identity you can maybe work around this.
Perhaps the built-in accounts have permissions to do the cache memory statistics gathering initiated by the pollingInterval that my custom Identity does not have, I don't know.
Hope this helps or even that someone else can join more of the dots to figure out a better work around.