FileDependency 文件更改后缓存失败

Question

我正在使用带有 FileDependency 的 Microsoft Enterprise Library Caching 库（版本 5）。

在我想要缓存的类上，我有一个静态属性，它要么从缓存中返回项目，要么创建一个新类并将其添加到缓存中。

这最初运行良好，并且类被创建一次，从那时起，缓存的副本被返回。但是，一旦依赖文件发生更改，缓存项就永远不会返回。

我在下面整理了一个示例程序来说明这个问题。

这个的输出是

999 cached , 1 uncached
999 cached , 1001 uncached

我希望结果是

 999 cached , 1 uncached
1998 cached , 2 uncached

看起来该对象已添加回缓存，但随后由于过期而立即被删除。

有什么想法吗？

using System;
using Microsoft.Practices.EnterpriseLibrary.Common.Configuration;
using Microsoft.Practices.EnterpriseLibrary.Caching;
using Microsoft.Practices.EnterpriseLibrary.Caching.Expirations;

namespace TestCache
{
  static class Program
  {
    [STAThread]
    static void Main()
    {
        Cache.Create();

        for (int i = 0; i < 1000; i++)
            TestClass.Current.DummyMethod();
        Console.WriteLine(String.Format("{0} cached , {1} uncached", TestClass.CachedItems, TestClass.UncachedItems));

        System.IO.File.AppendAllText(Cache.dependencyFileName, "Test");

        for (int i = 0; i < 1000; i++)
            TestClass.Current.DummyMethod();           
        Console.WriteLine(String.Format("{0} cached , {1} uncached", TestClass.CachedItems, TestClass.UncachedItems));

        Console.ReadLine();
    }
}

 public class Cache
{
    public static CacheManager cacheManager = null;
    public static string dependencyFileName;
    public static FileDependency objFileDependency;

    public static void Create()
    {
        var builder = new ConfigurationSourceBuilder();

        builder.ConfigureCaching()
               .ForCacheManagerNamed("TestCache")
               .UseAsDefaultCache()
               .StoreInMemory();

        var configSource = new DictionaryConfigurationSource();
        builder.UpdateConfigurationWithReplace(configSource);
        EnterpriseLibraryContainer.Current = EnterpriseLibraryContainer.CreateDefaultContainer(configSource);

        cacheManager = (CacheManager)EnterpriseLibraryContainer.Current.GetInstance<ICacheManager>("TestCache");

        dependencyFileName = "testCache.xml";
        if (!System.IO.File.Exists(dependencyFileName))
            using (System.IO.File.Create(dependencyFileName)) { }

        objFileDependency = new FileDependency(dependencyFileName);
    }
}

public class TestClass
{

    public static int CachedItems =0;
    public static int UncachedItems = 0;

    public void DummyMethod()
    {
    }

    public static TestClass Current
    {
        get
        {
            TestClass current = (Cache.cacheManager.GetData("Test") as TestClass);
            if (current != null)
                CachedItems++;
            else
            {
                UncachedItems++;
                current = new TestClass();
                Cache.cacheManager.Add("Test", current, CacheItemPriority.Normal, null, new ICacheItemExpiration[] { Cache.objFileDependency });
            }
            return current;
        }
    }

}

}

Answer 1

您的问题是您使用的是静态FileDependency。这导致 FileDependency 的 LastUpdateTime 永远不会被更新，进而导致添加到缓存中的所有项目都显示为过期 (HasExpired() == true)。即使您将项目添加到缓存中，因为它们已过期，您也永远无法检索它们。

解决方案是对缓存的所有添加使用新的FileDependency 对象。最简单的更改是用属性替换 objFileDependency 字段。使用您现有的名称和方法，代码如下所示：

public class Cache
{
    public static CacheManager cacheManager = null;
    public static readonly string dependencyFileName = "testCache.xml";            

    public static FileDependency objFileDependency
    {
        get
        {
            return new FileDependency(dependencyFileName);
        }
    }

    public static void Create()
    {
        var builder = new ConfigurationSourceBuilder();

        builder.ConfigureCaching()
               .ForCacheManagerNamed("TestCache")
               .UseAsDefaultCache()
               .StoreInMemory();

        var configSource = new DictionaryConfigurationSource();
        builder.UpdateConfigurationWithReplace(configSource);
        EnterpriseLibraryContainer.Current = EnterpriseLibraryContainer.CreateDefaultContainer(configSource);

        cacheManager = (CacheManager)EnterpriseLibraryContainer.Current.GetInstance<ICacheManager>("TestCache");

        if (!System.IO.File.Exists(dependencyFileName))
            using (System.IO.File.Create(dependencyFileName)) { }
    }
}