What is the best way to lock cache in asp.net?

Craig Shoemaker has made an excellent show on asp.net caching: http://polymorphicpodcast.com/shows/webperformance/

This article from CodeGuru explains various cache locking scenarios as well as some best practices for ASP.NET cache locking:

Synchronizing Cache Access in ASP.NET

Here's the basic pattern:

• Check the cache for the value, return if its available
• If the value is not in the cache, then implement a lock
• Inside the lock, check the cache again, you might have been blocked
• Perform the value look up and cache it
• Release the lock

In code, it looks like this:

private static object ThisLock = new object();


public string GetFoo()
{


// try to pull from cache here


lock (ThisLock)
{
// cache was empty before we got the lock, check again inside the lock


// cache is still empty, so retreive the value here


// store the value in the cache here
}


// return the cached value here


}

I saw one pattern recently called Correct State Bag Access Pattern, which seemed to touch on this.

I modified it a bit to be thread-safe.

http://weblogs.asp.net/craigshoemaker/archive/2008/08/28/asp-net-caching-and-performance.aspx

private static object _listLock = new object();


public List List() {
string cacheKey = "customers";
List myList = Cache[cacheKey] as List;
if(myList == null) {
lock (_listLock) {
myList = Cache[cacheKey] as List;
if (myList == null) {
myList = DAL.ListCustomers();
Cache.Insert(cacheKey, mList, null, SiteConfig.CacheDuration, TimeSpan.Zero);
}
}
}
return myList;
}

For completeness a full example would look something like this.

private static object ThisLock = new object();
...
object dataObject = Cache["globalData"];
if( dataObject == null )
{
lock( ThisLock )
{
dataObject = Cache["globalData"];


if( dataObject == null )
{
//Get Data from db
dataObject = GlobalObj.GetData();
Cache["globalData"] = dataObject;
}
}
}
return dataObject;

Just to echo what Pavel said, I believe this is the most thread safe way of writing it

private T GetOrAddToCache<T>(string cacheKey, GenericObjectParamsDelegate<T> creator, params object[] creatorArgs) where T : class, new()
{
T returnValue = HttpContext.Current.Cache[cacheKey] as T;
if (returnValue == null)
{
lock (this)
{
returnValue = HttpContext.Current.Cache[cacheKey] as T;
if (returnValue == null)
{
returnValue = creator(creatorArgs);
if (returnValue == null)
{
throw new Exception("Attempt to cache a null reference");
}
HttpContext.Current.Cache.Add(
cacheKey,
returnValue,
null,
System.Web.Caching.Cache.NoAbsoluteExpiration,
System.Web.Caching.Cache.NoSlidingExpiration,
CacheItemPriority.Normal,
null);
}
}
}


return returnValue;
}

I have come up with the following extension method:

private static readonly object _lock = new object();


public static TResult GetOrAdd<TResult>(this Cache cache, string key, Func<TResult> action, int duration = 300) {
TResult result;
var data = cache[key]; // Can't cast using as operator as TResult may be an int or bool


if (data == null) {
lock (_lock) {
data = cache[key];


if (data == null) {
result = action();


if (result == null)
return result;


if (duration > 0)
cache.Insert(key, result, null, DateTime.UtcNow.AddSeconds(duration), TimeSpan.Zero);
} else
result = (TResult)data;
}
} else
result = (TResult)data;


return result;
}

I have used both @John Owen and @user378380 answers. My solution allows you to store int and bool values within the cache aswell.

Please correct me if there's any errors or whether it can be written a little better.

I've wrote a library that solves that particular issue: Rocks.Caching

Also I've blogged about this problem in details and explained why it's important here.

I modified @user378380's code for more flexibility. Instead of returning TResult now returns object for accepting different types in order. Also adding some parameters for flexibility. All the idea belongs to @user378380.

 private static readonly object _lock = new object();




//If getOnly is true, only get existing cache value, not updating it. If cache value is null then      set it first as running action method. So could return old value or action result value.
//If getOnly is false, update the old value with action result. If cache value is null then      set it first as running action method. So always return action result value.
//With oldValueReturned boolean we can cast returning object(if it is not null) appropriate type on main code.




public static object GetOrAdd<TResult>(this Cache cache, string key, Func<TResult> action,
DateTime absoluteExpireTime, TimeSpan slidingExpireTime, bool getOnly, out bool oldValueReturned)
{
object result;
var data = cache[key];


if (data == null)
{
lock (_lock)
{
data = cache[key];


if (data == null)
{
oldValueReturned = false;
result = action();


if (result == null)
{
return result;
}


cache.Insert(key, result, null, absoluteExpireTime, slidingExpireTime);
}
else
{
if (getOnly)
{
oldValueReturned = true;
result = data;
}
else
{
oldValueReturned = false;
result = action();
if (result == null)
{
return result;
}


cache.Insert(key, result, null, absoluteExpireTime, slidingExpireTime);
}
}
}
}
else
{
if(getOnly)
{
oldValueReturned = true;
result = data;
}
else
{
oldValueReturned = false;
result = action();
if (result == null)
{
return result;
}


cache.Insert(key, result, null, absoluteExpireTime, slidingExpireTime);
}
}


return result;
}

There is no need to lock the whole cache instance, rather we only need to lock the specific key that you are inserting for. I.e. No need to block access to the female toilet while you use the male toilet :)

The implementation below allows for locking of specific cache-keys using a concurrent dictionary. This way you can run GetOrAdd() for two different keys at the same time - but not for the same key at the same time.

using System;
using System.Collections.Concurrent;
using System.Web.Caching;


public static class CacheExtensions
{
private static ConcurrentDictionary<string, object> keyLocks = new ConcurrentDictionary<string, object>();


/// <summary>
/// Get or Add the item to the cache using the given key. Lazily executes the value factory only if/when needed
/// </summary>
public static T GetOrAdd<T>(this Cache cache, string key, int durationInSeconds, Func<T> factory)
where T : class
{
// Try and get value from the cache
var value = cache.Get(key);
if (value == null)
{
// If not yet cached, lock the key value and add to cache
lock (keyLocks.GetOrAdd(key, new object()))
{
// Try and get from cache again in case it has been added in the meantime
value = cache.Get(key);
if (value == null && (value = factory()) != null)
{
// TODO: Some of these parameters could be added to method signature later if required
cache.Insert(
key: key,
value: value,
dependencies: null,
absoluteExpiration: DateTime.Now.AddSeconds(durationInSeconds),
slidingExpiration: Cache.NoSlidingExpiration,
priority: CacheItemPriority.Default,
onRemoveCallback: null);
}


// Remove temporary key lock
keyLocks.TryRemove(key, out object locker);
}
}


return value as T;
}
}