How to remove all objects (reset ) from IMemoryCache in ASP.NET core

Question

There is a Remove method to remove an object from IMemoryCache by its key. Is there a way to reset the whole cache and remove all objects?

Using the Dispose method as stated by How to clear MemoryCache? does not work:

ObjectDisposedException: Cannot access a disposed object.
 Object name: 'Microsoft.Extensions.Caching.Memory.MemoryCache'.

Answer 1

See Cache in-memory in ASP.NET Core, specifically the section on Cache dependencies.

Using a CancellationTokenSource allows multiple cache entries to be evicted as a group

This code worked for me:

public class CacheProvider 
{
    private static CancellationTokenSource _resetCacheToken = new CancellationTokenSource();
    private readonly IMemoryCache _innerCache;

    /* other methods and constructor removed for brevity */

    public T Set<T>(object key, T value) 
    {
        /* some other code removed for brevity */
        var options = new MemoryCacheEntryOptions().SetPriority(CacheItemPriority.Normal).SetAbsoluteExpiration(typeExpiration);
        options.AddExpirationToken(new CancellationChangeToken(_resetCacheToken.Token));

        _innerCache.Set(CreateKey(type, key), value, options);

        return value;
    }

    public void Reset()
    {
        if (_resetCacheToken != null && !_resetCacheToken.IsCancellationRequested && _resetCacheToken.Token.CanBeCanceled)
        {
            _resetCacheToken.Cancel();
            _resetCacheToken.Dispose();
        }

        _resetCacheToken = new CancellationTokenSource();
    }
}

Answer 2

The easiest way is Compact(1.0) if it's available. This code will clear the memory cache using an extension method (tested in unit tests and on production on .NET core 2.2 and 3.1). If Compact is not available, then fallback methods are used, starting with a public Clear method, followed by an internal Clear method. If none of those are available, an exception is thrown.

/// <summary>
/// Clear IMemoryCache
/// </summary>
/// <param name="cache">Cache</param>
/// <exception cref="InvalidOperationException">Unable to clear memory cache</exception>
/// <exception cref="ArgumentNullException">Cache is null</exception>
public static void Clear(this IMemoryCache cache)
{
    if (cache == null)
    {
        throw new ArgumentNullException("Memory cache must not be null");
    }
    else if (cache is MemoryCache memCache)
    {
        memCache.Compact(1.0);
        return;
    }
    else
    {
        MethodInfo clearMethod = cache.GetType().GetMethod("Clear", BindingFlags.Instance | BindingFlags.Public);
        if (clearMethod != null)
        {
            clearMethod.Invoke(cache, null);
            return;
        }
        else
        {
            PropertyInfo prop = cache.GetType().GetProperty("EntriesCollection", BindingFlags.Instance | BindingFlags.GetProperty | BindingFlags.NonPublic | BindingFlags.Public);
            if (prop != null)
            {
                object innerCache = prop.GetValue(cache);
                if (innerCache != null)
                {
                    clearMethod = innerCache.GetType().GetMethod("Clear", BindingFlags.Instance | BindingFlags.Public);
                    if (clearMethod != null)
                    {
                        clearMethod.Invoke(innerCache, null);
                        return;
                    }
                }
            }
        }
    }

    throw new InvalidOperationException("Unable to clear memory cache instance of type " + cache.GetType().FullName);
}

Answer 3

This code can help if you're using the standard MemoryCache. Documentation: https://docs.microsoft.com/en-us/aspnet/core/performance/caching/memory?view=aspnetcore-3.1#memorycachecompact

_cache.Compact(1.0);

Answer 4

My solution was to create a wrapper which re-expose existing few methods and add a missing method by replacing MemoryCache object with a brand new one. Worked just fine for me. Code is below:

public interface IMyMemoryCache : IMemoryCache
{
    void Reset();
}
public class MyMemoryCache: IMyMemoryCache
{
    IMemoryCache _memoryCache;

    public MyMemoryCache()
    {
        Reset();
    }
    public void Dispose()
    {
        _memoryCache.Dispose();
    }

    public bool TryGetValue(object key, out object value)
    {
        return _memoryCache.TryGetValue(key, out value);
    }

    public ICacheEntry CreateEntry(object key)
    {
        return _memoryCache.CreateEntry(key);
    }

    public void Remove(object key)
    {
        _memoryCache.Remove(key);
    }

    public void Reset()
    {
        var existingCache = _memoryCache;
        _memoryCache = new MemoryCache(new MemoryCacheOptions());

        // Dispose existing cache (we override) in 10 minutes
        if (existingCache != null)
        {
            System.Threading.Tasks.Task.Delay(TimeSpan.FromMinutes(10))
                .ContinueWith(t =>
                {
                    existingCache.Dispose();
                });
        }
    }
}

Answer 5

The answer as of RC1 is that you can't do it out of the box from what I've read and been told (I did read on GitHub that there maybe a way to create triggers to facilitate this that are coming).

Currently, you are provided Get, Set and Remove. I see your options as:

1. Create a cache manager wrapper that will track all of your keys, you can then remove those items in bulk as you see fit. I'm not in love with this but it would work. Of course, if you're not the one controlling the adding there could be things in the cache you are unaware of (you could compare your count to it's count to see). If you cast IMemoryCache as MemoryCache you can get the Count property which is exposed.
2. Fork the assembly and expose the Keys and/or add a method to remove those items. There is an underlying dictionary that holds the keys. I did this, compiled it, create a Nuget package for it and then replaced the RC1 version just to see if I could (and it worked). Not sure if this is the right way but here's the commit to my fork, I just added a read only property where I dumped the keys to an object list (the keys are stored as objects). As with past MemoryCache implementations, if you exposed the keys they could be stale after they're dumped, but if you're just using them to clear all then that shouldn't matter.

https://github.com/blakepell/Caching/commit/165ae5ec13cc51c44a6007d6b88bd9c567e1d724

I posted this issue last night trying to figure out if there's a good way to inspect what's in the cache specifically (asking why don't we have a way to). If you don't ask you'll never know if it would have mattered so I figured why not.

https://github.com/aspnet/Caching/issues/149

Answer 6

my solution was to set new expiration date to all the items in cache to 1 millisecond. Then they expired and hence cache flush.

Answer 7

I solved it by creating a FlushableMemoryCache singleton around IMemoryCache, which tracks the keys currently stored in the cache and then can just iterate over them to flush them all:

public interface IFlushableMemoryCache
{
    void Set<T>(string cacheId, object key, T value);
    bool TryGetValue<T>(object key, out T value);
    void Remove(string cacheId, object key);
    void Flush(string cacheId);
}


public class FlushableMemoryCache : IFlushableMemoryCache
{
    private readonly IMemoryCache _memoryCache;
    private readonly ConcurrentDictionary<string, HashSet<object>> _keyDictionary;

    public FlushableMemoryCache(IMemoryCache memoryCache)
    {
        _memoryCache = memoryCache;
        _keyDictionary = new ConcurrentDictionary<string, HashSet<object>>();
    }


    public void Set<T>(string cacheId, object key, T value)
    {
        _memoryCache.Set(key, value);

        _keyDictionary.AddOrUpdate(cacheId, new HashSet<object>(new[] {key}),
            (id, oldVal) =>
            {
                oldVal.Add(key);
                return oldVal;
            });
    }

    public bool TryGetValue<T>(object key, out T value)
    {
        return _memoryCache.TryGetValue(key, out value);
    }

    public void Remove(string cacheId, object key)
    {
        _memoryCache.Remove(key);

        if (_keyDictionary.ContainsKey(cacheId) && _keyDictionary[cacheId].Contains(key))
        {
            _keyDictionary[cacheId].Remove(key);
        }
    }

    public void Flush(string cacheId)
    {
        foreach (var key in _keyDictionary[cacheId])
        {
            _memoryCache.Remove(key);
        }

        _keyDictionary[cacheId] = new HashSet<object>();
    }
}

The services which make use of this will need to provide a cacheId which is unique to that service. That allows the Flush to only clear keys related to the specific service, and not everything in the cache!