Donut caching in asp.net mvc 2 with Html.Action April 8, 2010

[Update - ok, so this officially doesn't work, which sucks. I'm having a look at alternatives, but at the moment it looks like that is going to be reverting to MVC 1.0]

To do donut caching in Asp.Net Mvc 1.0 required quite a bit of hacking (or at least that was the only way I could get it to work), but with version 2 things get much better because of the addition of the Html.Action method.

As a quick recap, ‘donut caching’ is when you cache the whole of a web page except for small parts within it – the holes in the donut. A classic use for it is pages which have some personalised aspect like a logged in user name but are otherwise identical for all users.

To solve exactly this scenario I used the following code:

A child action called LoggedInUserInfo in my HomeController

        [ChildActionOnly]
        public ActionResult LoggedInUserInfo(HttpContextBase context)
        {
            return View(context);
        }

with a view of the same name:

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<HttpContextBase>" %>
<%@ Import Namespace="KableDirect.Service" %>
<%@ Import Namespace="KableDirect.Web.FrontEnd.Models.Shared"%>

    <ul>
        <li>You are logged in as <%= Model.User.Identity.Name %> </li>

        <li><%=Html.ActionLink("My Profile", "Profile", "Account")%></li>

        <%if (Model.User.IsAdmin()){%>
        <li><a href="<%= RouteHelper.GetAdminUrl(Model)%>">Admin</a></li>
        <%}%>
        <li><%=Html.ActionLink("Logout", "Logoff", "Account")%></li>
    </ul>

I also have the following HtmlHelper method to make calling the asp.net WriteSubstitution() method easier:

    public delegate string MvcCacheCallback(HttpContextBase context);
    public static class CacheHelper
    {
        public static object Substitute(this HtmlHelper html, MvcCacheCallback cb)
        {
            html.ViewContext.HttpContext.Response.WriteSubstitution(c => cb(new HttpContextWrapper(c)));
            return null;
        }
    }

Now I can call my child action within my MasterPage using Html.Action as follows:

<%=  Html.Substitute(context => Html.Action("LoggedInUserInfo", "Home", new {context}).ToString()) %>

It is important that your action takes the context as a parameter or it will only be called once the first time the page is hit and then get cached along with everything else.
It is also important that you have a route in your route table that will catch the {controller}/{action} url pattern. See this post for more details.

Phill Haack now has a warning on his site about problems with donut caching in Asp.Net Mvc 2, but this has worked fine for me, I’ll update here if I find scenarios where this doesn’t work.

There is a good introduction to Html.Action here.

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