C# - How To Implement VaryByCustom Caching

I'm trying to implement functionality to cache certain pages depending on the host. This is because I can have multiple versions of a page which have the same parameters, and where the only difference in terms of a request is the host that is being requested. So, for example these two URLs will request the same page, but they are styled differently:[URL]and [URL]


I'm going through the example outlined here:[URL]but it's not making sense to me.

I've added this to my global.asax:

public override string GetVaryByCustomString(HttpContext context, string arg)
{
if (arg == "host")
{
return "host=" + context.Request.Url.Host;
}
return base.GetVaryByCustomString(context, arg);
}

and the example states "To set the custom string programmatically, call the SetVaryByCustom method and pass it the custom string to use", with code similar to the following:

Response.Cache.SetVaryByCustom("host");

The problem is I'm not sure what to do with this. I've added the previous line to MvcApplication_EndRequest because it seems like it makes sense, but I don't think this is right because when I set breakpoints in GetVaryByCustomString they never get hit.

[Code]....