缓存(销毁)依赖

1:缓存依赖的类别
      文件,数据表,数据行,一段时间,控件,查询字符串等
2:缓存项之间的依赖

protected void Button3_Click(object sender, EventArgs e)
{
    // create item A and item B
    string itemA = "ItemA";
    string itemB = "ItemB";
    Cache.Insert("ItemA", itemA, null, DateTime.Now.AddMinutes(10), 
    TimeSpan.Zero,
    CacheItemPriority.Default, MyItemRemovedCallBack);
    Cache.Insert("ItemB", itemB, null, DateTime.Now.AddMinutes(10), 
    TimeSpan.Zero,
    CacheItemPriority.Default, MyItemRemovedCallBack);
}


private void MyItemRemovedCallBack(string key, object value,
                   CacheItemRemovedReason reason)
{
    // remove the item from the cache
    if (key == "ItemA" ) Cache.Remove("ItemB");
    else if (key.Equals == "ItemB") Cache.Remove("ItemB");
}

以上代码中
Key:缓存集合中唯一确认一项的键
Value:保存在缓存中的值
Reason:是"CacheItemRemovedReason"的一个枚举.说明缓存项被移出的原因
我们也可以手动移出缓存

Cache.Remove("ItemA");

3: 缓存依赖于文件

<?xml version="1.0" encoding="utf-8"?>
<MenuItems>
    <MenuItem>
        <Text>Home</Text>
    </MenuItem>
    <MenuItem>
        <Text>About us</Text>
    </MenuItem>
    <MenuItem>
        <Text>Contact us</Text>
    </MenuItem>
    <MenuItem>
        <Text>Help</Text>
    </MenuItem>
    <MenuItem>
        <Text>Feature</Text>
    </MenuItem>
</MenuItems>



    string menuPath = "MyFiles/Menu.xml";
    string folderName = "MyFiles/";

    DataSet ds = null;
    if (Cache["Menu"] == null)
    {
        ds = new DataSet();
        ds.ReadXml(Server.MapPath(menuPath));

        // menu is created
        Cache.Insert("Menu", ds, new System.Web.Caching.CacheDependency(
            Server.MapPath(menuPath)),DateTime.Now.AddMinutes(60),
            TimeSpan.Zero,
            System.Web.Caching.CacheItemPriority.Default,
            new System.Web.Caching.CacheItemRemovedCallback(
                                  CacheItemRemovedCallBack));

       DisplayCacheCreationTime("Object was not in the cache and created at:",
             DateTime.Now.ToLongTimeString());
    }
    else
    {
        // menu is created from the cache
        DisplayCacheCreationTime("Object was in the cache",String.Empty);
    }

我们也可以将缓存依赖于文件夹.当文件夹下的文件和子级文件夹被创建,更新,删除的时候跟新缓存.
4:数据库中的缓存依赖

<connectionStrings>
    <add name="ConnectionString"
         connectionString="Server=localhost;Database=School;
Trusted_Connection=true"/>
</connectionStrings>

<system.web>
    <caching>
        <sqlCacheDependency pollTime="10000" enabled="true" >
            <databases>
                <add connectionStringName="ConnectionString" name="School"/>
            </databases>
        </sqlCacheDependency>
    </caching></caching>


    if (Cache["Users"] == null)
    {
        // Create the cache dependency
        SqlCacheDependency dep = new SqlCacheDependency("School", "Users");
        string connectionString = ConfigurationManager.ConnectionStrings[
                                        "ConnectionString"].ConnectionString;
        SqlConnection myConnection = new SqlConnection(connectionString);
        SqlDataAdapter ad = new SqlDataAdapter("SELECT FirstName, LastName " +
                                               "FROM Users", myConnection);
        DataSet ds = new DataSet();
        ad.Fill(ds);

        // put in the cache object
        Cache.Insert("Users", ds, dep);
    }

    gvUsers.DataSource = Cache["Users"] as DataSet;
    gvUsers.DataBind();

SqlCacheDependency dep = new SqlCacheDependency("School", "Users"); 这句话用于创建一个与数据库School中表Users关联的缓存.当我们对Users进行INSERT, DELETE, UPDATE操作的时候缓存会被移出(Cache["Users"]==null ).
这部分代码适用于sql2000,在sql2005下没有这么麻烦.回头我们再讨论sql2005下的缓存.