You are aware your code will not compile as is ?
Consider that it might be difficult/impossible to match exactly in C# the internal algorithms and data-structures used by Windows Calc ... unless MS published them.
You could, using the WinAPI Platform Invoke facilities use 'SetParent, and 'MoveWindow, to kind-of "embed" the Win Calculator in your .NET C# app, but I suspect that would be unsatisfactory, and inter-operation with it ... as in reading the contents of the display text-edit window ... is ... tricky.
MS has an example of using 'SendKeys in C# to control the Win Calculator: [
^].
The .NET NameSpace System.Windows.Automation is a better way to attempt to inter-operate; see: [
^].