ASP.NET Cache緩存的使用

       ASP.NET Cache是提高系統性能的重要方法，它使用了「最近使用」原則（a least-recently-used algorithm）。在數據庫訪問中常常會用到Cache保存數據庫數據。

1.緩存的添加：

Cache的添加方法有Add()或Insert（），兩種方法幾乎相似，只是Inser方法能夠使用可選參數，即便用默認參數，來實現緩存的添加：

Cache.Add(

       KeyName,//緩存名

       KeyValue,//要緩存的對象

       Dependencies,//依賴項

       AbsoluteExpiration,//絕對過時時間

       SlidingExpiration,//相對過時時間

       Priority,//優先級

       CacheItemRemovedCallback);//緩存過時引起事件

2. 緩存依賴項：

       緩存能夠設置的時效性能夠經過 文件依賴，其餘緩存依賴，數據庫依賴和過時時間方法來設置，當文件改變，依賴緩存項改變，數據庫改變或時間的到期時，緩存會失效，並能夠引起必定事件。

2.1 文件依賴：

        CacheDependency fileDepends = new CacheDependency(Server.MapPath("Northwind.xml"));

        Cache.Insert("GridViewDataSet", dsGrid, fileDepends);

此例爲經過Northiwind.xml文件依賴出來緩存的用法：

2.2 其餘緩存項依賴：

string[] fileDependsArray = {Server.MapPath("Northwind.xml")};

string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};

CacheDependency cacheDepends = new CacheDependency(fileDependsArray, cacheDependsArray);

Cache.Insert("GridViewDataSet", dsGrid, cacheDepends);

此例設置了Northwind.xml文件依賴和 Depend0,depend1,Depend2緩存項

其中Depend0,depend1,Depend2爲另外三個緩存。

若是不須要文件依賴能夠設置爲NULL。 

2.3 過時時間設定：

AbsoluteExpiration能夠設置緩存的絕對過時時間，如：

Cache.Insert("GridViewDataSet ", dsGrid, null, DateTime.Now.AddMinutes(30), Cache.NoSlidingExpiration);

緩存會在添加起30分鐘後過時。

NoSlidingExpiration能夠設置相對過時時間,若是緩存在NoSlidingExpiration設定的時間內沒有被訪問，緩存過時，若是在這段時間內有訪問，則緩存過時時間將會重置爲原始值，如NoSlidingExpiration=20

在20分鐘內若是沒有被訪問，緩存過時，若是每次19分鐘訪問緩存，緩存將永遠不會過時。

Cache.Insert("DataGridDataSet", dsGrid, null,Cache.NoAbsoluteExpiration, TimeSpan.FromSeconds(30));

3. 優先級：

        Priority屬性值和意義：

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. 緩存失效事件處理：

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.

 

出處：http://www.cnblogs.com/leochu2008/articles/1161772.html