CrystalB is not quite right - it is possible to get the result from Calc - it uses standard controls so as long as the user hasn't closed it, it is possible if you follow the window handles and locate it's control ID.
But...I've never done it, partly because it's a PITA from C# and partly because it is a nasty and rather stupid thing to do.
Instead, I would look at the various calculator apps available (and there are loads:
Codeproject calculators[
^]) and adapt one of those to work within your application and return results when you want them to. It'd probably be easier, and certainly would be safer, given that Calc.exe is subject to change at the whim of Microsoft.
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