I am developing an application using Win32 GetOpenFileName win32 API. When the window opened even if I click cancel memory usage incredibly increases from 2 mb to 8 mb. What could be the reason of this behaviour?

Thanks in advance.

Most likely it's something else in your prograsm. Comment out the call to GetOpenFileName() then re-run your test to see what happens. My guess is that the program will still display the same behavior.

Unfortunately not. Every time I run the memory usage increases. 8-14-19 and so on :(

I would suspect your program contains memory leaks. win32 api functions do not exhibit that type of problem.