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Maybe I misinterpreted regarding changing the interface..

When the interface of a DLL had changed, It causes to create a new ClassID?
If yes, do we need to set the project reference again?

' Specifying vbNormalFocus as the second argument opens the application in
' normal size and gives it the focus.
Dim RetVal
RetVal = Shell("C:\WINDOWS\prg2.EXE 0001", vbNormalFocus) ' Run prg2....

I think, I cannot call the event handler because it is two different applications not two form within an application (two different applications means two executable file e.g. PRG1.exe , PRG2.exe).
...

How about my favor on number 2? is there other way to automatically link the newly created ClassID via batch file?

It is very Informative.

I just followed what you had said, and everything were successfully done.

I created a batch file to compile all the DLLs and Projects.

Problem:
1) I need to add...

Yes, I strongly agree with you. But Activate event has its own limitation. The said event is not called if between two applications involve.

Please teach me.

Yes i strongly agree with you but there is always an exemptions. Please read msdn site if you have time.
http://msdn2.microsoft.com/en-us/library/aa445402(VS.60).aspx

I have 2 exe programs.
prg1.exe = calling program
prg2.exe = program to be called

prg1 code to call prg2:
' Specifying vbNormalFocus as the second argument opens the application in
' normal...