VaryByHeader with OutputCacheAttribute on child actions

Question

With the [OutputCacheAttribute] in ASP.NET MVC 3, you can output cache with a good deal of flexibility. It's useful to leverage the 'VaryByHeader' property to bucket caching by host name. For example:

[OutputCache(Duration = 60, VaryByHeader = "host")]
public ActionResult Foo()
{
    return this.View();
}

However, on child actions, you cannot apply the 'VaryByHeader'. The framework throws the following exception:

OutputCacheAttribute for child actions only supports Duration, VaryByCustom, and VaryByParam values. Please do not set CacheProfile, Location, NoStore, SqlDependency, VaryByContentEncoding, or VaryByHeader values for child actions.

My question is, why?

Is the reason why we cannot VaryByHeader in a child action because it would provide a conflicting variance, since the parent action might have specified a different VaryByHeader value?

If I want to cache child actions differently based upon hostname, what does this mean, and how would I go about it?

Answer 1

VaryByHeader affects the actual HTTP Response headers; so you're probably correct that the MVC team blocked this to prevent conflicts with the parent action.

To cache based on hostname, couldn't you use VaryByCustom? Something like (disclaimer: haven't tried this at all):

[OutputCache(Duration = 60, VaryByCustom = "host")]
public ActionResult Foo()
{
    return View();
}

followed by (in your Global.asax.cs)

public override string GetVaryByCustomString(HttpContext context, string arg)
{
    if (arg == "host")
    {
        return context.Request.Headers["host"];
    }

    // whatever you have already, or just String.Empty
    return String.Empty;
}