Cache

ASP.NET Cache是提升系统性能的重要方法，它使用了“最近使用”原则（a least-recently-used algorithm）。在数据库访问中经常会用到Cache保存数据库数据。

1.缓存的添加：

Cache的添加方法有Add()或Insert（），两种方法几乎类似，只是Inser方法可以使用可选参数，即使用默认参数，来实现缓存的添加：

Cache.Add(

       KeyName,//缓存名

       KeyValue,//要缓存的对象

       Dependencies,//依赖项

       AbsoluteExpiration,//绝对过期时间

       SlidingExpiration,//相对过期时间

       Priority,//优先级

       CacheItemRemovedCallback);//缓存过期引发事件

2. 缓存依赖项：

       缓存可以设置的时效性可以通过 文件依赖，其他缓存依赖，数据库依赖和过期时间方法来设置，当文件改变，依赖缓存项改变，数据库改变或时间的到期时，缓存会失效，并可以引发一定事件。

2.1 文件依赖：

        CacheDependency fileDepends = new CacheDependency(Server.MapPath("Northwind.xml"));
        Cache.Insert("GridViewDataSet", dsGrid, fileDepends);
此例为通过Northiwind.xml文件依赖出来缓存的用法：
2.2 其他缓存项依赖：
string[] fileDependsArray = {Server.MapPath("Northwind.xml")};
string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};
CacheDependency cacheDepends = new CacheDependency(fileDependsArray, cacheDependsArray);
Cache.Insert("GridViewDataSet", dsGrid, cacheDepends);
此例设置了Northwind.xml文件依赖和 Depend0,depend1,Depend2缓存项
其中Depend0,depend1,Depend2为另外三个缓存。
如果不需要文件依赖可以设置为NULL。 
2.3 过期时间设定：
AbsoluteExpiration可以设置缓存的绝对过期时间，如：
Cache.Insert("GridViewDataSet ", dsGrid, null, DateTime.Now.AddMinutes(30), Cache.NoSlidingExpiration);
缓存会在添加起30分钟后过期。
NoSlidingExpiration可以设置相对过期时间,如果缓存在NoSlidingExpiration设定的时间内没有被访问，缓存过期，如果在这段时间内有访问，则缓存过期时间将会重置为原始值，如NoSlidingExpiration=20
在20分钟内如果没有被访问，缓存过期，如果每次19分钟访问缓存，缓存将永远不会过期。
Cache.Insert("DataGridDataSet", dsGrid, null,Cache.NoAbsoluteExpiration, TimeSpan.FromSeconds(30));
3. 优先级：
        Priority属性值和意义：
Priority value
 Description
 
NotRemovable
 Items with this priority will not be evicted.
 
High
 Items with this priority level are the least likely to be evicted.
 
AboveNormal
 Items with this priority level are less likely to be evicted than items assigned Normal priority.
 
Default
 This is equivalent to Normal.
 
Normal
 The default value.
 
BelowNormal
 Items with this priority level are more likely to be evicted than items assigned Normal priority.
 
Low
 Items with this priority level are the most likely to be evicted.
 

 
4. 缓存失效事件处理：

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Web.Caching;         // necessary for CacheDependency
using System.Xml;                  // necessary for Xml stuff
 
public partial class _Default : System.Web.UI.Page
{
   public static CacheItemRemovedCallback onRemove = null;
 
   protected void Page_Load(object sender, EventArgs e)
    {
       CreateGridView( );
    }
 
    private void CreateGridView( )
    {
       DataSet dsGrid;
       dsGrid = (DataSet)Cache["GridViewDataSet"];
 
       onRemove = new CacheItemRemovedCallback(this.RemovedCallback);
 
       if (dsGrid == null)
       {
          dsGrid = GetDataSet( );
          string[] fileDependsArray = {Server.MapPath("Northwind.xml")};
          string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};
          CacheDependency cacheDepends = new CacheDependency
                                (fileDependsArray, cacheDependsArray);
          Cache.Insert("GridViewDataSet", dsGrid, cacheDepends,
                        DateTime.Now.AddSeconds(10),
                        Cache.NoSlidingExpiration,
                        CacheItemPriority.Default,
                        onRemove);
          lblMessage.Text = "Data from XML file.";
       }
       else
       {
          lblMessage.Text = "Data from cache.";
       }
 
       gv.DataSource = dsGrid.Tables[0];
       gv.DataBind( );
    }
 
    private DataSet GetDataSet( )
    {
       DataSet dsData = new DataSet( );
       XmlDataDocument doc = new XmlDataDocument( );
       doc.DataSet.ReadXml(Server.MapPath("Northwind.xml"));
       dsData = doc.DataSet;
       return dsData;
    }
 
   public void RemovedCallback(string cacheKey,
                                 Object cacheObject,
                                 CacheItemRemovedReason reasonToRemove)
   {
      WriteFile("Cache removed for following reason: " +
         reasonToRemove.ToString( ));
   }
 
   private void WriteFile(string strText)
   {
      System.IO.StreamWriter writer = new System.IO.StreamWriter(
                                                   @"C:"test.txt", true);
      string str;
      str = DateTime.Now.ToString( ) + " " + strText;
      writer.WriteLine(str);
      writer.Close( );
   }
 
   protected void btnClear_Click(object sender, EventArgs e)
    {
      Cache.Remove("GridViewDataSet");
      CreateGridView( );
    }
 
   protected void btnInit_Click(object sender, EventArgs e)
   {
      // Initialize caches to depend on.
      Cache["Depend0"] = "This is the first dependency.";
      Cache["Depend1"] = "This is the 2nd dependency.";
      Cache["Depend2"] = "This is the 3rd dependency.";
   }
 
   protected void btnKey0_Click(object sender, EventArgs e)
   {
      Cache["Depend0"] = "This is a changed first dependency.";
   }
}
Table 17-5. Members of the CacheItemRemovedReason enumeration
 
Reason
 Description
 
DependencyChanged
 A file or item key dependency has changed.
 
Expired
 The cached item has expired.
 
Removed
 The cached item has been explicitly removed by the Remove method or replaced by another item with the same key.
 
Underused
 The cached item was removed to free up system memory.