如何清除内存缓存？

Question

I have created a cache using the MemoryCache class. I add some items to it but when I need to reload the cache I want to clear it first. What is the quickest way to do this? Should I loop through all the items and remove them one at a time or is there a better way?

Answer 1

Dispose现有的 MemoryCache 并创建一个新的 MemoryCache 对象。

Answer 2

The problem with enumeration

The MemoryCache.GetEnumerator() Remarks section warns: "Retrieving an enumerator for a MemoryCache instance is a resource-intensive and blocking operation. Therefore, the enumerator should not be used in production applications."

Here's why , explained in pseudocode of the GetEnumerator() implementation:

Create a new Dictionary object (let's call it AllCache)
For Each per-processor segment in the cache (one Dictionary object per processor)
{
    Lock the segment/Dictionary (using lock construct)
    Iterate through the segment/Dictionary and add each name/value pair one-by-one
       to the AllCache Dictionary (using references to the original MemoryCacheKey
       and MemoryCacheEntry objects)
}
Create and return an enumerator on the AllCache Dictionary

Since the implementation splits the cache across multiple Dictionary objects, it must bring everything together into a single collection in order to hand back an enumerator. Every call to GetEnumerator executes the full copy process detailed above. The newly created Dictionary contains references to the original internal key and value objects, so your actual cached data values are not duplicated.

The warning in the documentation is correct. Avoid GetEnumerator() -- including all of the answers above that use LINQ queries.

A better and more flexible solution

Here's an efficient way of clearing the cache that simply builds on the existing change monitoring infrastructure. It also provides the flexibility to clear either the entire cache or just a named subset and has none of the problems discussed above.

// By Thomas F. Abraham (http://www.tfabraham.com)
namespace CacheTest
{
    using System;
    using System.Diagnostics;
    using System.Globalization;
    using System.Runtime.Caching;

    public class SignaledChangeEventArgs : EventArgs
    {
        public string Name { get; private set; }
        public SignaledChangeEventArgs(string name = null) { this.Name = name; }
    }

    /// <summary>
    /// Cache change monitor that allows an app to fire a change notification
    /// to all associated cache items.
    /// </summary>
    public class SignaledChangeMonitor : ChangeMonitor
    {
        // Shared across all SignaledChangeMonitors in the AppDomain
        private static event EventHandler<SignaledChangeEventArgs> Signaled;

        private string _name;
        private string _uniqueId = Guid.NewGuid().ToString("N", CultureInfo.InvariantCulture);

        public override string UniqueId
        {
            get { return _uniqueId; }
        }

        public SignaledChangeMonitor(string name = null)
        {
            _name = name;
            // Register instance with the shared event
            SignaledChangeMonitor.Signaled += OnSignalRaised;
            base.InitializationComplete();
        }

        public static void Signal(string name = null)
        {
            if (Signaled != null)
            {
                // Raise shared event to notify all subscribers
                Signaled(null, new SignaledChangeEventArgs(name));
            }
        }

        protected override void Dispose(bool disposing)
        {
            SignaledChangeMonitor.Signaled -= OnSignalRaised;
        }

        private void OnSignalRaised(object sender, SignaledChangeEventArgs e)
        {
            if (string.IsNullOrWhiteSpace(e.Name) || string.Compare(e.Name, _name, true) == 0)
            {
                Debug.WriteLine(
                    _uniqueId + " notifying cache of change.", "SignaledChangeMonitor");
                // Cache objects are obligated to remove entry upon change notification.
                base.OnChanged(null);
            }
        }
    }

    public static class CacheTester
    {
        public static void TestCache()
        {
            MemoryCache cache = MemoryCache.Default;

            // Add data to cache
            for (int idx = 0; idx < 50; idx++)
            {
                cache.Add("Key" + idx.ToString(), "Value" + idx.ToString(), GetPolicy(idx));
            }

            // Flush cached items associated with "NamedData" change monitors
            SignaledChangeMonitor.Signal("NamedData");

            // Flush all cached items
            SignaledChangeMonitor.Signal();
        }

        private static CacheItemPolicy GetPolicy(int idx)
        {
            string name = (idx % 2 == 0) ? null : "NamedData";

            CacheItemPolicy cip = new CacheItemPolicy();
            cip.AbsoluteExpiration = System.DateTimeOffset.UtcNow.AddHours(1);
            cip.ChangeMonitors.Add(new SignaledChangeMonitor(name));
            return cip;
        }
    }
}

Answer 3

From http://connect.microsoft.com/VisualStudio/feedback/details/723620/memorycache-class-needs-a-clear-method

The workaround is:

List<string> cacheKeys = MemoryCache.Default.Select(kvp => kvp.Key).ToList();
foreach (string cacheKey in cacheKeys)
{
    MemoryCache.Default.Remove(cacheKey);
}

Answer 4

var cacheItems = cache.ToList();

foreach (KeyValuePair<String, Object> a in cacheItems)
{
    cache.Remove(a.Key);
}

Answer 5

如果性能不是问题，那么这个漂亮的单线将解决问题：

cache.ToList().ForEach(a => cache.Remove(a.Key));

Answer 6

It seems that there is a Trim method.

So to clear all contents you'd just do

cache.Trim(100)

EDIT: after digging some more, it seems that looking into Trim is not worth your time

https://connect.microsoft.com/VisualStudio/feedback/details/831755/memorycache-trim-method-doesnt-evict-100-of-the-items

How do I clear a System.Runtime.Caching.MemoryCache

Answer 7

Ran across this, and based on it, wrote a slightly more effective, parallel clear method:

    public void ClearAll()
    {
        var allKeys = _cache.Select(o => o.Key);
        Parallel.ForEach(allKeys, key => _cache.Remove(key));
    }

Answer 8

You could also do something like this:

Dim _Qry = (From n In CacheObject.AsParallel()
           Select n).ToList()
For Each i In _Qry
    CacheObject.Remove(i.Key)
Next

Answer 9

You can dispose the MemoryCache.Default cache and then re-set the private field singleton to null, to make it recreate the MemoryCache.Default.

       var field = typeof(MemoryCache).GetField("s_defaultCache",
            BindingFlags.Static |
            BindingFlags.NonPublic);
        field.SetValue(null, null);

Answer 10

I was only interested in clearing the cache and found this as an option, when using the c# GlobalCachingProvider

                var cache = GlobalCachingProvider.Instance.GetAllItems();
                if (dbOperation.SuccessLoadingAllCacheToDB(cache))
                {
                    cache.Clear();
                }

Answer 11

a bit improved version of magritte answer.

var cacheKeys = MemoryCache.Default.Where(kvp.Value is MyType).Select(kvp => kvp.Key).ToList();
foreach (string cacheKey in cacheKeys)
{
    MemoryCache.Default.Remove(cacheKey);
}

Answer 12

This discussion is also being done here: https://docs.microsoft.com/en-us/answers/answers/983399/view.html

I wrote an answer there and I'll transcribe it here:

using System.Collections.Generic;
using Microsoft.Extensions.Caching.Memory;
using ServiceStack;

public static class IMemoryCacheExtensions
{
    static readonly List<object> entries = new();

    /// <summary>
    /// Removes all entries, added via the "TryGetValueExtension()" method
    /// </summary>
    /// <param name="cache"></param>
    public static void Clear(this IMemoryCache cache)
    {
        for (int i = 0; i < entries.Count; i++)
        {
            cache.Remove(entries[i]);
        }
    }

    /// <summary>
    /// Use this extension method, to be able to remove all your entries later using "Clear()" method
    /// </summary>
    /// <typeparam name="TItem"></typeparam>
    /// <param name="cache"></param>
    /// <param name="key"></param>
    /// <param name="value"></param>
    /// <returns></returns>
    public static bool TryGetValueExtension<TItem>(this IMemoryCache cache, object key, out TItem value)
    {
        entries.AddIfNotExists(key);

        if (cache.TryGetValue(key, out object result))
        {
            if (result == null)
            {
                value = default;
                return true;
            }

            if (result is TItem item)
            {
                value = item;
                return true;
            }
        }

        value = default;
        return false;
    }
}