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Caching objects

While the OutputCache directive is easy to use and work with, it allows you only to cache the entire page, or an entire UserControl. While this is just fine for some situations, there will be times where caching an object will be much smarter. While the following example might seem a bit silly, it does show how the Cache object can be used to store for instance an ArrayList with custom objects in it. Create a new project, or use an existing page and change the CodeBehind Page_Load method to something like this (this example doesn't require you to modify the markup part):

protected void Page_Load(object sender, EventArgs e)
    ArrayList datestamps;
    if(Cache["datestamps"] == null)
        datestamps = new ArrayList();

        Cache.Add("datestamps", datestamps, null, System.Web.Caching.Cache.NoAbsoluteExpiration, new TimeSpan(0, 0, 60), System.Web.Caching.CacheItemPriority.Default, null);
        datestamps = (ArrayList)Cache["datestamps"];

    foreach(DateTime dt in datestamps)
        Response.Write(dt.ToString() + "<br />");

Try running this example and reloading the page. 3 instances of the current date is placed in an ArrayList, and the list is then cached using the Cache object. On each page load, the Cache is checked to see if it contains our ArrayList, using a name we have provided. If it's not, the list is generated, filled and cached. If the ArrayList is already in the cache, we pull it out and cast it to the correct type (everything is stored as object types). No matter where we get the list from, we iterate through it and output each value.

Most of it should be pretty straight forward, except for the line where we add the list to the Cache. The Add method takes a bunch of parameters, and I will try to explain all of them.

First of all, the key we wish to store our object by, in this case we simply call it "datestamps". The key is used to get the object back from the Cache.

Next parameter is the object we wish to cache, so that goes without explaining.

Next up is the CacheDependency parameter. It can be used to tell ASP.NET that the cached item depends on other items, allowing ASP.NET to invalidate the cache if one of these items changes.

The following two parameters tells ASP.NET how and when the item should expire in the cache. The first one specifies an absolute expiration, meaning that the item will expire at a certain time of day. This is usually not what you want, so we set it to NoAbsoluteExpiration. Next we can specify a sliding expiration, which means that the item will expire after a certain amount of time, based on when the item is placed in the cache. The TimeSpan class is used, and we specify that it should expire after 0 hours, 0 minutes and 60 seconds. A priority can be set for the item in the cache. This is used when ASP.NET is removing items from the cache - items with a lower priority are removed first.

The last parameter allows us to specify a callback function for the item, which will be called once the item is removed from the cache, in case you need that. We simply specify a null value.

So, as you can see, storing items by using the Cache object is really easy. Of course, we don't really earn any performance in this example, but only the imagination is the limit here. One of the most common practices are storing objects retrieved from a database in the cache, so save resources on the database machine and allow for faster loading pages.

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