Using the HttpCachePolicy Class to Enable Output Caching
As previously mentioned, the @OutputCache directive is derived from the HttpCachePolicy class, which gives you lower-level access to the output cache. The HttpCachePolicy class is exposed through the HttpResponse.Cache property; and it is a member of the System.Web namespace. You would want to use the HttpCachePolicy rather than the @OutputCache when you need to dynamically set the output cache settings. For instance, if you want to dynamically set how long the item should remain in the cache.
The @OutputCache directive exposes nearly all the functionality available from using the HttpCachePolicy class, but the class does offer more functionality than @OutputCache. For instance, when using the @OutputCache directive, you can only set the cache expiration time in seconds. The HttpCachePolicy class enables you to set either an absolute—DateTime.Parse ("11:59:00PM") or a relative expiration time—DateTime.Now.AddMinutes(2).
Because I went over nearly all the attributes for the HttpCachePolicy class in the @OutputCache directive section, I will not go into great detail about each here. However, I will provide a code example of implementing caching using the HttpCachePolicy class in Listing 16.10.
Listing 16.10. Implementing Output Caching Using the HttpCachePolicy Class
[VisualBasic.NET]
1: <script language="vb" runat="server" >
2:
3: public sub Page_Load(sender as Object, e as EventArgs)
4:
5: Response.Cache.SetExpires(DateTime.Now.AddMinutes(2))
6: Response.Cache.SetCacheability(HttpCacheability.Public)
7:
8: end sub
9:
10: </script>
[C#.NET]
01: <script language="c#" runat="server" >
02:
03: void Page_Load(Object sender, EventArgs e) {
04:
05: Response.Cache.SetExpires(DateTime.Now.AddMinutes(2));
06: Response.Cache.SetCacheability(HttpCacheability.Public);
07:
08: }
9:
10: </script>
[VisualBasic.NET & C#.NET]
11: <html>
12: <body>
13: <b>This page was cached at</b>: <%=DateTime.Now.ToString("G")%>
14: </body>
15: </html>
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In Listing 16.10, caching is enabled on lines 5 and 6. On line 5, the HttpCachePolicy.Set Expires method sets the cache duration (SetExpires will be discussed in the following section). On line 6, the SetCacheability method sets the cache to public visibility, passing in a member of the HttpCacheability enumeration. (These objects are covered in the following section.)
The following list details members of the HttpCachePolicy class. I will not give a full code example for each member in this section because I went over the effect of using these members in the @OutputCache directive section, just under a different context.
SetExpires— Used to set an absolute date and time that the cache shall expire. In Listing 16.8 I used DateTime.Now.AddMinutes(2) as the expiration time, which takes the current time and adds 2 minutes. Alternatively, you can put in a hard-coded value such as "12:00:00AM".ToDateTime(). This is comparable to the @OutputCache's Duration attribute, but takes an absolute date and time rather than a value of seconds.
SetCacheability— Used to control where documents are cached, whether only on the server or on both clients and the server. This is similar to the @OuputCache's Location attribute. The value passed into this method must be a member of the HttpCachePolicy enumeration (a member of System.Web). A list and description of each value follows:
SetMaxAge— Used to set the maximum time span the cache item should be considered valid. The parameter for this method should be a TimeSpan type, such as
Response.Cache.SetMaxAge(TimeSpan)
SetNoServerCaching— Used to restrict any caching from taking place on the server that processed the request. This method expects no parameters:
Response.Cache.SetNoServerCaching()
SetSlidingExpiration— Used to turn sliding expiration on or off. This method expects a Boolean value as a parameter:
Response.Cache.SetSlidingExpiration(true | false)
SetVaryByCustom— Used to set the VaryByCustom string. This method expects a string as a parameter (for example, Platform):
Response.Cache.SetVaryByCustom(string)
HttpCacheVaryByHeaders— Class used to control the cache to vary by a Header(s).
HttpCacheVaryByParams— Class used to control the cache to vary by a parameter(s).