Setting CustomProperty "isReference" according to Aggregation Type properly

[sar2]

Hello,

In my code I'm creating Elements with parts, which are connected by Aggregation, both type 1 and 2. However when creating a Diagram I had to notice that my parts are all displayed as "properties".
Is there a way to create my Part objects correctly, so they are displayed as "references" when Aggregation type is share in my code? I tried setting the CustomProperty value of "isReference", but it did not update when I checked in the GUI.

Thank you in advance.