How do I read the Icon\CR file?

The subject says - nearly - all…

I am talking about the custom icon in the Finder.

You know, the file whose size is 0 Bytes… and is invisible (hidden by the Finder).

I suppose this is not possible without Declare…

Yup, afriad so: Declares or plugins, as you need to access Apple API that isn’t included as part of the Xojo framework.

Custom icon of a folderitem can be read:

dim iconsize as integer
dim p as picture = folderitem.IconMBS(iconsize)

I failed when I wrote the question…

The important part is… the file length is 0 Bytes. That means the icon in in Resources.

And in the 2012r2 Release Notes, we can read:
** CHANGE 20698 Mac: the ResourceFork class has been removed.*

Then ResourceForkMBS could help.