We use quite a lot of blocks spread across our site.

These blocks are basically outputting title, image and link based on the child nodes of a pagereference property set in the block's properties.

I'd like to cache each instance of these blocks, and have been playing around with the asp.net outputcache (I've also tried caching just the data, e.g. the pagecollection fetched from a block's pagereference property, but the performance gain was a lot smaller compared to caching the entire usercontrol).

Since a block type may be used on multiple pages and also multiple times on the same page, I figured VaryByControl was the best suited OutputCacheParameters to use. That way, I were hoping that a cached version of the block based on its block properties would be created. 

So I did like this:

    [PartialCaching(50000, null, "NewsBlock.AllParams", null, false)]
    public partial class NewsBlock: SiteBlockControlBase
    {
        public string AllParams
        {
            get { return String.Format("{0};{1}",
                CurrentBlock.ListLink.ID,
                CurrentBlock.SkipCount.HasValue ? CurrentBlock.SkipCount.Value : 0
                );
            }
        }
...
}

And in my SiteBlockControlBase base class, I invalidate the cache when content has been changed on the site (via DataFactoryCache.VersionKey):

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            BasePartialCachingControl pcc = Parent as BasePartialCachingControl;
            if (pcc != null) pcc.Dependency = new CacheDependency(null, new[]
            {                
                DataFactoryCache.VersionKey
            });
        }

My problem is - that sporadically, a block cached for one page, may show up on another page (where the same block type, but not instance, is used). So it seems like the block properties are being ignored, and the cached version of the block/usercontrol are served regardless.

I'm questioning if maybe there is some EPiServer "magic" happening behind the scenes that is does not support caching of blocks? Maybe outputcaching of blocks is not supported for this kind of use-case?

Any suggestions are appreciated!