Hi Avishek;
1) Mutli-threading in PB would only be done using NVUO's not Window classes.
2) In order to communicate between the main thread and the sub-thread - you need to only use NVUO's.
3) Here is a high level architecture diagram as to how multi-threading would be done in a typical PB Classic application.
Note1: The sub-thread handler is also an NVUO
Note2: The StartThread ( ) method would use the SharedObjectRegister ( ) PB method.
Note3: The main thread NVUO must pass the handle of itself to the sub-thread.
Note4: The Thread Handler can only "POST" information to the SubThread.
Note5: The SubThread can only "POST" information back to the MainThread via its SubThread Handler. and therefore must have its Handle to do so.
HTH
Regards ... Chris