This works:
public override void OnActionExecuting(ActionExecutingContext filterContext)
{
var httpContext = filterContext.RequestContext.HttpContext;
string eTag = httpContext.Request.Headers["ETag"];
string responseETag = GetEtag();
if (!string.IsNullOrEmpty(eTag))
{
if (eTag.Equals(responseETag))
{
filterContext.HttpContext.Response.StatusCode = 304;
filterContext.HttpContext.Response.StatusDescription = "Not Modified";
}
return;
}
httpContext.Response.AddHeader("ETag", responseETag);
}