Several pages of an upcoming Mix Online lab are fundamentally static, except for a single hyperlink that toggles depending on whether a user is logged in or not.  To optimize the site, I wanted to use caching on these static pages. But by adding the OutputCache attribute to these pages, a user could end up getting a cached page with the wrong logged-in hyperlink: users who weren’t logged in would see a page that stated they were logged in—or vice versa.

The solution? Using the VaryByCustom parameter of the OutputCache attribute, so that both a logged-in and a logged-out version of the page are cached and served up appropriately. Here’s how to do it:

First, add a method to the global.asax.cs file, which overrides the VaryByCustom call:

    public override string GetVaryByCustomString(HttpContext context,
    string arg)
    {
        if (arg == "IsLoggedIn")
        {
             if (context.Request.Cookies["anon"] != null)
             {
                  if (context.Request.Cookies["anon"].Value == "false")
                  {
                       return "auth";
                  }
                  else
                  {
                       return "anon";
                  }
              }
              else
              {
                 return "anon";
              }
        }
        else
        {
            return base.GetVaryByCustomString(context, arg);
        }

    }

Then, decorate the controller actions with the OutputCache attribute:

    [OutputCache(CacheProfile = "StaticPage")]
    public ActionResult Index()
    {
       return View();
    }

When using caching, I like to keep all the profile information in web.config. This lets me update things without recompiling. Here’s what the cache profile looks like:

   <caching>
        <outputcachesettings>                         <outputcacheprofiles>
                <clear />
                <!-- 24 hours-->
                <add varybycustom="IsLoggedIn" varybyparam="*" duration="86400" name="StaticPage" />
            </outputcacheprofiles>
        </outputcachesettings>
    </caching>

Notice how the varybyparam attribute is set with an asterisk, and the varybycustom attribute has the string IsLoggedIn,the term that gets passed to the VaryByCustom override.

I’m using my own kind of authentication (more on that in a future post),so checking a cookie is sufficient. If you’re using ASP.NET Membership Authentication, you can use slightly different logic such that pages are cached based on the username itself. See this post for a sample—and tip o’ the hat to Chris Mullins for setting me on this path!