@Tee_
normally a pro file results in a separate target (executable or library). Even if you "combine" them with a subdirs project.
So your AppUi project must link against the AppLib

@rtavakko I'm so pleased to hear the working solution! Thanks for sharing the good news :)

Hi,

What are these "minor things" ?

After few days searching on Google.
I found the answer.

The problem is the order you Link to Static Library.

Dependency of my Projects is:
Presentation << Utilities << DynamicObjectCore

in Presentation.pro, I'm link to Static Link Library in this order

# Link to DynamicObjectCore ... # Link to Utilities ...

I thought that it will compile DynamicObjectCore first, then compile Utilities.
But it wan't, and cause the compile error. "undefined reference to..."

When i change the Link Order

# Link to Utilities ... # Link to DynamicObjectCore ...

And it work.