如何清除MemoryCache？

Question

我已经使用MemoryCache类创建了一个缓存。我向其中添加了一些项，但当我需要重新加载缓存时，我想首先清除它。完成此操作的最快方法是什么？我应该遍历所有的项目，一次删除一个项目，还是有更好的方法？

Answer 1

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

Answer 2

枚举的问题

MemoryCache.GetEnumerator() Remarks section警告：“检索MemoryCache实例的枚举器是一项资源密集型和阻塞操作。因此，不应在生产应用程序中使用枚举器。”

这里是的原因，在GetEnumerator()实现的伪代码中解释：

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

由于该实现跨多个Dictionary对象拆分缓存，因此它必须将所有内容合并到一个集合中，以便返回枚举器。每次调用GetEnumerator都会执行上面详细介绍的完整复制过程。新创建的Dictionary包含对原始内部键和值对象的引用，因此实际缓存的数据值不会重复。

文档中的警告是正确的。避免使用GetEnumerator() --包括上面使用LINQ查询的所有答案。

更好、更灵活的解决方案

这里有一种清除缓存的有效方法，它只需构建在现有的更改监视基础设施上。它还提供了清除整个缓存或仅清除命名子集的灵活性，并且不存在上面讨论的任何问题。

// 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

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

解决方法是：

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