ASP.NET MVC Bundling cache. (Detecting css files changes) (internal behaviour)

The ASP.NET Optimization framework caches the bundle response in HttpContext.Cache and uses a CacheDependency to monitor each file in the bundle for changes. This is why updating the files directly invalidates the cache and regenerates the bundle.

The bundle file name is a hash of the bundle contents which ensures the URL changes when any of the bundle files are modified. The bundle's virtual path is used as the cache key.

The relevant code from the library (note this is slightly out of date but I believe the logic is still the same):

internal BundleResponse GetBundleResponse(BundleContext context)
{
    // check to see if the bundle response is in the cache
    BundleResponse bundleResponse = Bundle.CacheLookup(context);
    if (bundleResponse == null || context.EnableInstrumentation)
    {
        // if not, generate the bundle response and cache it
        bundleResponse = this.GenerateBundleResponse(context);
        if (context.UseServerCache)
        {
            this.UpdateCache(context, bundleResponse);
        }
    }
    return bundleResponse;
}

private void UpdateCache(BundleContext context, BundleResponse response)
{
    if (context.UseServerCache)
    {
        // create a list of all the file paths in the bundle
            List<string> list = new List<string>();
        list.AddRange(
            from f in response.Files
            select f.FullName);
        list.AddRange(context.CacheDependencyDirectories);
        string cacheKey = Bundle.GetCacheKey(context.BundleVirtualPath);
        // insert the response into the cache with a cache dependency that monitors
        // the bundle files for changes
        context.HttpContext.Cache.Insert(cacheKey, response, new CacheDependency(list.ToArray()));
        context.HttpContext.Response.AddCacheItemDependency(cacheKey);
        this._cacheKeys.Add(cacheKey);
    }
}

Finally as for old bundle URLs working, I think you will find they are either returned from your browser cache or actually return the latest version of the bundle since the bundle path doesn't change, only the version query string.