Thank you, Tony.
I believe the documentCollection.DocumentActivated, which defined in IExtensionApplication function of Dll B, should run add first when Dll B got loaded in AutoCAD;
the one defined in a palette of Dll A, which referenced in Dll B, will only be loaded when a command called, so it never could load before the one in Dll B.
In this case, my code is safe.
Thank you and Merry Christamas
Wes
Windows 10 64 bit, AutoCAD (ACA, Map) 2023