Introducing Cache Dependencies:
As time passes, the data source may change in response to other actions. However, if your code uses caching, you may remain unaware of the changes and continue using out-of-date information from the cache. To help mitigate this problem, ASP.NET supports cache dependencies. Cache dependencies allow you to make a cached item dependent on another resource so that when that resource changes the cached item is removed automatically. ASP.NET includes three types of dependencies:

• Dependencies on other cache items.
• Dependencies on files or folders.
• Dependencies on a database query.

Cache Notifications in SQL Server 2000:
Before you can use SQL Server cache invalidation, you need to enable notifications for the database. This task is performed with the aspnet_regsql.exe command-line utility, which is located in the c:\[WinDir]\Microsoft.NET\Framework\[Version] directory. To enable notifications, you need to use the -ed command-line switch. You also need to identify the server (use -E for a trusted connection and -S to choose a server other than the current computer) and the database (use -d). Here's an example that enables notifications for the Northwind database on the current server:

aspnet_regsql -ed -E -d

NorthwindAfter executing this command, a new table named SqlCacheTablesForChangeNotification is added to the database Northwind. The SqlCacheTablesForChangeNotification table has three columns: tableName, notificationCreated, and changeId. This table is used to track changes. Essentially, when a change takes place, a record is written into this table. The SQL Server polling queries this table. Also a set of stored procedures is added to the database.

How Notificaions Works:
The AspNet_SqlCacheTablesForChangeNotification contains a single record for every table you're monitoring. When you make a change in the table (such as inserting ,deleting or updating a record), the changeId column is incremented by 1 -see AspNet_SqlCacheUpdateChangeIdStoredProcedure procedure-. ASP.NET queries this table repeatedly and keeps track of the most recent changeId values for every table. When this value changes in a subsequent read, ASP.NET knows that the table has changed.

In this scenario, Any change to the table is deemed to invalidate any query for that table. In other words, if you use this query:

SELECT * FROM Products WHERE CategoryID=1

The caching still works in the same way. That means if any product record is touched, even if the product resides in another category (and therefore isn't one of the cached records), the notification is still sent and the cached item is considered invalid. Also keep in mind it doesn't make sense to cache tables that change frequently.

Enable ASP.Net Polling:
To enable ASP.NET polling , you need to use the element in the web.config file. Set the enabled attribute to true to turn it on, and set the pollTime attribute to the number of milliseconds between each poll. (The higher the poll time, the longer the potential delay before a change is detected.) You also need to supply the connection string information.

Creating the Cache Dependency:
Now that we've seen how to set up a database to support SQL Server notifications, the only remaining detail is the code, which is quite straightforward. We can use our cache dependency with programmatic data caching, a data source control, and output caching.
For programmatic data caching, we need to create a new SqlCacheDependency and supply that to the Cache.Insert() method. In the SqlCacheDependency constructor, you supply two strings. The first is the name of the database you defined in the element in the section of the web.config file e.g: Northwind. The second is the name of the linked table e.g: Products.Example:private static void CacheProductsList(List products){
SqlCacheDependency sqlDependency = new SqlCacheDependency("Northwind", "Products");HttpContext.Current.Cache.Insert("ProductsList", products, sqlDependency, DateTime.Now.AddDays(1), Cache.NoSlidingExpiration);
}
private static List GetCachedProductList(){
return HttpContext.Current.Cache["ProductsList"] as List;
}
NWProductItem is business class, and here we are trying to cache a list of NWProductItem instead of DataSet or DataTable.The following method is used by an ObjectDataSource Control to retrieve List of Products
public static List GetProductsList(int catId, string sortBy){
//Try to Get Products List from the CacheList products = GetCachedProductList();if (products == null){
//Products List not in the cache, so we need to query the Database by using a Data LayerNWProductsDB db = new NWProductsDB(_connectionString);DbDataReader reader = null;products = new List(80);
if (catId > 0){
//Return Product List from the Data Layerreader = db.GetProductsList(catId);
}else{
//Return Product List from the Data Layerreader = db.GetProductsList();
}//Create List of Products -List if NWProductItem-products = BuildProductsList(reader);reader.Close();//Add entry to products list in the CacheCacheProductsList(products);
}products.Sort(new NWProductItemComparer(sortBy));if (sortBy.Contains("DESC")) products.Reverse();return products;
}
To perform the same trick with output caching, you simply need to set the SqlDependency property with the database dependency name and the table name, separated by a colon: <%@ OutputCache Duration="600" SqlDependency="Northwind:Products" VaryByParam="none" %>

<%@ OutputCache Duration="600" SqlDependency="Northwind:Products" VaryByParam="none" %>The same technique works with the SqlDataSource and ObjectDataSource controls: