progress bar filling

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C#

I have a problem in filling the progress bar

i am calling the "vc dll" in c#. The input function takes the
input file path and out put file path.

i want to fill my progress bar during the time when dll called and on path, out put file is created.
how to fill the progress bar in c# during the time the dll function called, and out put file created.

plz help

Regards