如何实现VaryByCustom缓存？

<caching>
 <outputCacheSettings>
   <outputCacheProfiles>
      <add name="CustomVaryBy" varyByCustom="CustomVaryBy" duration="60" location="Server" />
    </outputCacheProfiles>
  </outputCacheSettings>
</caching>

protected void Application_BeginRequest(Object sender, EventArgs e)
{
    HttpContext.Current.Response.AddCacheItemDependency("CustomVaryBy");
}

public override string GetVaryByCustomString(HttpContext context, string custom)
{
    if (custom == "CustomVaryBy")
    {
        // 在这里定义自定义变量
        return "CustomVaryByValue";
    }
    return base.GetVaryByCustomString(context, custom);
}

<%@ OutputCache Duration="60" VaryByCustom="CustomVaryBy" %>