Using OutputCache with PartialView in MVC 3, with programmatic invalidation usingVaryByCustom too!

I've had a fun morning trying to cache partial views inside a dynamic page with MVC 3. Specifically, a partial view in a razor layout page (i.e. masterpage) that shows some dynamically generated data which I would like to hold off from regenerating for 60 seconds or so, even if the user refreshes or navigates around the site.

So I create my partial view like this - here we generate the time to show when the view is being cached

MyPartial.cshtml

Timestamp: @DateTime.Now.ToLongTimeString()

Then I create the controller action:

HomeController.cs

[OutputCache(Duration = 60)]
public ActionResult MyPartial()
{
    return PartialView();
}

Then I amend my layout page to include the call to the PartialView:

MyMaster.cshtml

Some header stuff
...
@Html.Action("MyPartial", "Home")
...
Some footer stuff

Well, that's all there is to it. The pitfalls I faced revolved around accidentally calling Html.Partial() from the layout page, which didn't work because it bypasses the controller attributes (i.e. it misses the OutputCache directive).

HOWEVER

Then I started wanting to reset the output cache when the underlying data changed, or when some user setting changed. So I knocked up this little solution using the OutputCache attribute's VaryByCustom property.

First a modification to the controller:

HomeController.cs

[OutputCache(Duration = 60, VaryByCustom = "WebApp.OutputCacheKey")]
public ActionResult MyPartial()
{
    return PartialView();
}

Every time OutputCache is called, it will check to see if that string has changed since last time. If it has changed, it invalidates the cache. "But it's a static string! How can it change?" I hear you cry. Yup, here's some voodoo magic. You can override a special method in global.asax that intercepts the call and returns ANOTHER string, wooo!

global.asax

public override string GetVaryByCustomString(HttpContext context, string arg)
{
    if (arg == "WebApp.OutputCacheKey")
    {
        return WebApp.OutputCacheKey;
    }
    else
    {
        return base.GetVaryByCustomString(context, arg);
    }
}

Now, we need to implement this WebApp thing. It's just an approach I like to have a static class knocking around my web application that allows me to access stuff from one central place, like strongly-typed accessors for session variables, or my per-request EF datacontext. But for these purposes it would just look like this:

WebApp.cs

public static class WebApp
{
    internal static string OutputCacheKey
    {
        get
        {
            if (HttpContext.Current.Session["WebApp.OutputCacheKey"] == null)
            {
                ResetOutputCache();
            }
            return HttpContext.Current.Session["WebApp.OutputCacheKey"].ToString();
        }
    }

    internal static void ResetOutputCache()
    {
        HttpContext.Current.Session["WebApp.OutputCacheKey"] = Guid.NewGuid().ToString();
    }    
}

So, what we have now is an easy way to programatically tell the OutputCache to clear itself, with a simple call from anywhere in your controller code:

WebApp.ResetOutputCache();

Simples!