API Callback made by External Application

I'm trying hard to use EM_STREAMOUT Example provided by someone else on this board to "steal" the RTF code of a Rich Text Box.

It works fine if I try it with the RTB of my own, but if I try it with other RTB of other application, a Protection Error occurs and it forces that applicaton to be shutdown.

I think it's due to sharing volation error, which occurs when that external application try to call MY Callback function in my program space.

Could I do something to grant that application to call my function?
Or any other way to get the RTF Code without using callback function?

Thanks in advance.