试图从多个缓存条目中引用 CacheDependency 对象

试图从多个缓存条目中引用 CacheDependency 对象
An attempt was made to reference a CacheDependency object from more than one Cache entry

代码一:

AggregateCacheDependency XmlFileCacheDependencyForBirds = new AggregateCacheDependency();
        XmlFileCacheDependencyForBirds.Add(
           new CacheDependency(HttpContext.Current.Server.MapPath("App_Data/Product.xml")),
           new CacheDependency(HttpContext.Current.Server.MapPath("App_Data/Category.xml")));
        HttpRuntime.Cache.Add("cacheKey1", "cacheData1", XmlFileCacheDependencyForBirds, 
            DateTime.MaxValue, new TimeSpan(1, 1, 1), CacheItemPriority.High, null);
        HttpRuntime.Cache.Add("cacheKey2", "cacheData2", XmlFileCacheDependencyForBirds, 
            DateTime.MaxValue, new TimeSpan(1, 1, 1), CacheItemPriority.High, null);

代码二:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Caching;
using System.Xml;


/// <summary>
/// Summary description for CacheManager
/// </summary>
public class TestCacheManager
{
    private static bool _productsRemovedFromCache = false;
    //记录访问从xml读取数据的相关信息
    private static Dictionary<string, string> _logs = new Dictionary<string, string>();
    public static Dictionary<string, string> Logs
    {
        get { return _logs; }
    }

    private static string productxmlPath;
    private static string categoryxmlPath;

    private static AggregateCacheDependency XmlFileCacheDependencyForBirds = new AggregateCacheDependency();
    private static AggregateCacheDependency XmlFileCacheDependencyForBugs = new AggregateCacheDependency();  

    static TestCacheManager()
    {
        productxmlPath = HttpContext.Current.Server.MapPath("App_Data/Product.xml");
        categoryxmlPath = HttpContext.Current.Server.MapPath("App_Data/Category.xml");

        XmlFileCacheDependencyForBirds.Add(
            new CacheDependency(HttpContext.Current.Server.MapPath("App_Data/Product.xml")),
            new CacheDependency(HttpContext.Current.Server.MapPath("App_Data/Category.xml")));

        XmlFileCacheDependencyForBugs.Add(
            new CacheDependency(HttpContext.Current.Server.MapPath("App_Data/Product.xml")),
            new CacheDependency(HttpContext.Current.Server.MapPath("App_Data/Category.xml")));
    }

    private static AggregateCacheDependency GetAggregateCacheDependency(string categoryKey)
    {
        switch (categoryKey)
        {
            case "BIRDS":
                return XmlFileCacheDependencyForBirds;
            case "BUGS":
                return XmlFileCacheDependencyForBugs;
            default:
                return null;
        }
    }

    //从缓存中获取项  
    public static List<Product> GetProductsByCategory(string categoryKey)
    {
        //不应在 ASP.NET 页中实现回调处理程序，因为在从缓存中删除项之前该页可能已被释放，因此用于处理回调的方法将不可用，应该在非ASP.NET的程序集中实现回调处理程序。为了确保从缓存中删除项时处理回调的方法仍然存在，请使用该方法的静态类。但是，静态类的缺点是需要保证所有静态方法都是线程安全的，所以使用lock关键字。
        lock (typeof(CacheManager))
        {
            if (HttpContext.Current.Cache[categoryKey] != null)
            {    //存在缓存项，返回缓存值   
                return (List<Product>)HttpRuntime.Cache[categoryKey];
            }
            else
            {   //缓存项不存在，则创建缓存项  
                CacheProducts(categoryKey);
                return (List<Product>)HttpRuntime.Cache[categoryKey];
            }
        }
    }

    //将项以  的名称添加到缓存中，并将该项设置为在添加到缓存中后一分钟过期。  //并且该方法注册 ReportRemoveCallback 方法，以便在从缓存中删除项时进行调用。   
    public static void CacheProducts(string categoryKey)
    {
        lock (typeof(CacheManager))
        {
            //To avoid error that An attempt was made to reference a CacheDependency object from more than one Cache entry.
            //AggregateCacheDependency acd = new AggregateCacheDependency();
            //acd.Add(
            //    new CacheDependency(productxmlPath),
            //    new CacheDependency(categoryxmlPath));

            HttpRuntime.Cache.Add(
                categoryKey,
                CreateProducts(categoryKey),
                //acd,
                //An attempt was made to reference a CacheDependency object from more than one Cache entry.
                GetAggregateCacheDependency(categoryKey),                  
                DateTime.MaxValue,
                new TimeSpan(1, 1, 1),
                System.Web.Caching.CacheItemPriority.Default,
                //使用委托,在缓存删除时调用特定函数来响应。
                ProductsRemovedCallback);
        }
    }

    //创建报告，该报告时缓存项的值  
    private static List<Product> CreateProducts(string categoryKey)
    {
        List<Product> lstProduct = new List<Product>();
        XmlDocument doc = new XmlDocument();
        //HttpContext.Current.Server.MapPath("App_Data/Product.xml") Cache 失效后触发此函数时，HttpContext.Current 可能为Null
        doc.Load(productxmlPath);
        XmlNodeList productNodeList = doc.SelectNodes(string.Format("Products/Product[@CategoryId='{0}']", categoryKey));
        foreach (XmlNode productNode in productNodeList)
        {
            Product pro = new Product(productNode.Attributes["ProductId"].Value.ToString()
                , productNode.Attributes["Name"].Value.ToString()
                , productNode.Attributes["Descn"].Value.ToString()
                , productNode.Attributes["Image"].Value.ToString()
                , productNode.Attributes["CategoryId"].Value.ToString());
            lstProduct.Add(pro);
        }
        _logs.Remove(categoryKey);
        _logs.Add(categoryKey, string.Format("{1} (Access xml file to get product {0}'s data)", categoryKey, DateTime.Now.ToString()));
        return lstProduct;
    }

    /// <summary>
    /// 当从缓存中删除项时调用该方法  
    /// </summary>
    /// <param name="key">CacheKey</param>
    /// <param name="value">Cache Data</param>
    /// <param name="removedReason">Removed/Expired/Underused/DependencyChanged</param> 
    public static void ProductsRemovedCallback(String key, object value,
        CacheItemRemovedReason removedReason)
    {
        _productsRemovedFromCache = true;
        CacheProducts(key);
    }
}

代码一 vs. 代码二 运行是都将抛出 "An attempt was made to reference a CacheDependency object from more than one Cache entry" 的异常。

原因:每个CacheDependency 只与一个缓存项关联，要跟多个缓存项关联，只需要用相关参数新建CacheDependency对象即可。

解决方法: 只要确保每个CacheDependency 只与一个缓存项关联即可。 建议构建CacheDependency 、 Cache关联CacheDependency能够在同一单元里完成。类似如下:

// Create a AggregateCacheDependency object from the factory
                AggregateCacheDependency cd = DependencyFacade.GetItemDependency();

                // Store the output in the data cache, and Add the necessary AggregateCacheDependency object
                HttpRuntime.Cache.Add(key, data, cd, DateTime.Now.AddHours(itemTimeout), Cache.NoSlidingExpiration, CacheItemPriority.High, null);

---------------------------------------------------------------------------
AggregateCacheDependency acd = new AggregateCacheDependency();
            acd.Add(
                new CacheDependency(productxmlPath),
                new CacheDependency(categoryxmlPath));

            HttpRuntime.Cache.Add(
                categoryKey,
                CreateProducts(categoryKey),
                acd,
                //An attempt was made to reference a CacheDependency object from more than one Cache entry.
                //GetAggregateCacheDependency(categoryKey),                  
                DateTime.MaxValue,
                new TimeSpan(1, 1, 1),
                System.Web.Caching.CacheItemPriority.Default,
                //使用委托,在缓存删除时调用特定函数来响应。
                ProductsRemovedCallback);