'VB Cache("Greeting") = "Hello, world!" If Not (Cache("Greeting") Is Nothing) Then in VS .NET

Encode Quick Response Code in VS .NET 'VB Cache("Greeting") = "Hello, world!" If Not (Cache("Greeting") Is Nothing) Then

value = CType(Cache("Greeting"), String)

value = "Hello, world!"

End If //C# Cache["Greeting"] = "Hello, world!"; if (Cache["Greeting"] != null)

value = (string)Cache["Greeting"];

value = "Hello, world!";

Of course, you wouldn t normally cache a static string; you d normally cache a file or an object that s based on a relatively slow database or network query. You can cache any type object, but you must cast it to the correct type when accessing it. The previous example demonstrates that you can use the Cache object just like a stan dard collection, as long as you verify that the object being accessed isn t null. You can access much more sophisticated functionality, however, by using the Add and Insert methods. Each of these methods enables you to automatically remove an item from the cache after a specific period of time, or when a file, database object, or another cache object expires. To use the Cache.Insert method, provide the following parameters (parameters vary depending on the overload used):

A key The String that you ll use to access the cached object in the Cache collection. An Object to be cached

A dependency (optional)

A CacheDependency object that identifies a file or cache key, and that, when the file or cache key is changed, triggers this object to be removed from the cache. If you cache a file, you should configure a dependency for the file so that it is removed from cache after being modified, ensuring that your cache never becomes stale. This can be null.

An absolute expiration date (optional)

The time (in a DateTime object) at which the object should be removed from the cache, regardless of whether it has been recently accessed. Set this to System.Web.Caching.Cache.NoAbsoluteExpiration if you don t want to use it.

A time span (optional)

The time span (in a TimeSpan object) after which the object should be removed from the cache if it has not been accessed. Set this to System.Web.Caching.Cache.NoSlidingExpiration if you don t want to use it.

A priority (optional)

A CacheItemPriority enumeration value that you can use to determine which objects are removed first when memory starts to run low. Lowerpriority objects are removed sooner. Set this to System.Web.Caching.CacheItemPriority.Default if you don t want to use it. An event handler that is called when the object is removed from the cache. This can be null if you don t want to specify a callback method.

A callback method (optional)

For example, the following code sample demonstrates how to make a cache depen dency based on a file. If the file changes, the object is removed from cache.

'VB Cache.Insert("FileCache", "CacheContents", New System.Web.Caching.CacheDependency( _ Server.MapPath("SourceFile.xml"))) //C# Cache.Insert("FileCache", "CacheContents", new System.Web.Caching.CacheDependency( Server.MapPath("SourceFile.xml")));

You can also create multiple dependencies for a single object. The following example demonstrates how to use an AggregateCacheDependency object to add an item to the cache that is dependent on both an item named CacheItem1 and a file named SourceFile.xml.

'VB Dim dep1 As CacheDependency = New CacheDependency(Server.MapPath("SourceFile.xml"))

Dim keyDependencies2 As String() = {"CacheItem1"}

Dim dep2 As CacheDependency = New System.Web.Caching.CacheDependency(Nothing, _

keyDependencies2) Dim aggDep As AggregateCacheDependency = New System.Web.Caching.AggregateCacheDependency()

13

aggDep.Add(dep1)

aggDep.Add(dep2)

Cache.Insert("FileCache", "CacheContents", aggDep)

//C# System.Web.Caching.CacheDependency dep1 = new System.Web.Caching.CacheDependency(Server.MapPath("SourceFile.xml")); string[] keyDependencies2 = { "CacheItem1" }; System.Web.Caching.CacheDependency dep2 = new System.Web.Caching.CacheDependency(null, keyDependencies2);

new System.Web.Caching.AggregateCacheDependency(); aggDep.Add(dep1); aggDep.Add(dep2); Cache.Insert("FileCache", "CacheContents", aggDep);

If you want a cached object to be used for a specific amount of time because it will become outdated, pass the Cache.Insert method for a time in the future at which the object should expire. The DateTime.Now object has a variety of methods for adding a specific number of minutes to the current time, as the following example demonstrates:

'VB Cache.Insert("FileCache", "CacheContents", Nothing, DateTime.Now.AddMinutes(10.0), _ TimeSpan.Zero) //C# Cache.Insert("FileCache", "CacheContents", null, DateTime.Now.AddMinutes(10d), TimeSpan.Zero);

If you want your most frequently used cached objects to stay in your cache longer, specify a time span. The time span is the time after the last read request that the cached object will be retained. This example shows you how to keep an object in cache for five minutes after the last request:

'VB Cache.Insert("CacheItem7", "Cached Item 7", _ Nothing, System.Web.Caching.Cache.NoAbsoluteExpiration, New TimeSpan(0, 10, 0)) //C# Cache.Insert("CacheItem7", "Cached Item 7", null, System.Web.Caching.Cache.NoAbsoluteExpiration, new TimeSpan(0, 10, 0));

The Cache.Add method works exactly the same as the Cache.Insert method, except that it returns the defined value. That makes the Cache.Add method easier to use when you want to add an item to a cache and also defines another variable with a sin gle line of code.