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：

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 = http://www.mamicode.com/new DataSet( );

       XmlDataDocument doc = new XmlDataDocument( );

       doc.DataSet.ReadXml(Server.MapPath("Northwind.xml"));

       dsData = http://www.mamicode.com/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.";

   }

}