使用HostFileChangeMonitor的CacheItemPolicy需要时间来清理缓存

Question

当我使用CacheItemPolicy with HostFileChangeMonitor时，更改的文件需要5-7毫秒的缓存时间才能删除项目

[TestMethod]
public void TestHostFileChangeMonitor()
{
    var cachedFilePath = @"c:\temp\123.txt";
    File.WriteAllText(cachedFilePath, "1111");
    System.Runtime.Caching.ObjectCache cache = MemoryCache.Default;
    string fileContents = cache["filecontents"] as string;
    if (fileContents == null)
    {
        CacheItemPolicy policy = new CacheItemPolicy();
        List<string> filePaths = new List<string>();
        filePaths.Add(cachedFilePath);

        policy.ChangeMonitors.Add(new
            HostFileChangeMonitor(filePaths));

        // Fetch the file contents.  
        fileContents = File.ReadAllText(cachedFilePath);
        cache.Set("filecontents", fileContents, policy);
    }
    File.WriteAllText(cachedFilePath, "2222");
    int sleepTime = 5;
    Thread.Sleep(sleepTime);
    string fileContents2 = cache["filecontents"] as string;
    Assert.AreEqual("2222", File.ReadAllText(cachedFilePath));
    Assert.IsNull(fileContents2);// the test past only if  sleepTime > 5 
}

Answer 1

我的解决方案是将设置到缓存的时间保存在缓存和读取检查File.GetLastWriteTime中

[TestMethod]
public void TestHostFileChangeMonitor1()
{
    var cachedFilePath = @"c:\temp\123.txt";
    File.WriteAllText(cachedFilePath, "1111");
    System.Runtime.Caching.ObjectCache cache = MemoryCache.Default;
    CashObj cashObj = cache["filecontents"] as CashObj;
    if (cashObj == null || cashObj.addedTime < File.GetLastWriteTime(cachedFilePath))
    {
        CacheItemPolicy policy = new CacheItemPolicy();
        List<string> filePaths = new List<string>();
        filePaths.Add(cachedFilePath);

        policy.ChangeMonitors.Add(new
            HostFileChangeMonitor(filePaths));

        // Fetch the file contents.  
        var fileContents = File.ReadAllText(cachedFilePath);
        cache.Set("filecontents", new CashObj(fileContents), policy);
    }
    File.WriteAllText(cachedFilePath, "2222");
    cashObj = cache["filecontents"] as CashObj;
    Assert.AreEqual("2222", File.ReadAllText(cachedFilePath));
    Assert.IsTrue(cashObj == null || cashObj.addedTime < File.GetLastWriteTime(cachedFilePath));

}

public class CashObj
{
    public object data;
    public DateTime addedTime;

    public CashObj(object data)
    {
        this.data = data;
        this.addedTime = DateTime.Now;
    }
}

Answer 2

您可以等待项目逐出。例如，使用Microsoft.Extensions.Caching.Memory.MemoryCache，您可以使用下一种方法来完成此操作。

        TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();
        string key = "key";

        MemoryCache msCache = new MemoryCache(new MemoryCacheOptions());
        msCache.Set(key, tcs, new MemoryCacheEntryOptions()
            .RegisterPostEvictionCallback(
            (object callbackKey, object callbackValue, EvictionReason reason, object state) =>
            {
                tcs.SetResult(true);
            }));

        Task evictionTask = tcs.Task;

        msCache.Remove(key);

        await evictionTask;

同样，对于System.Runtime.MemoryCache，您可以使用CacheEntryRemovedCallback而不是RegisterPostEvictionCallback