how to shutdown a computer through code in c#...

There are a number of ways to do this.

The quickest solution is to use Process.Start("shutdown","/s 0");

For managed code, try WMI classes.
E.g.
ManagementClass mcWin32 = new ManagementClass("Win32_OperatingSystem");
Then invoke Win32Shutdown and shutdown the system.
You would need certain priveleges to access the method above.


For a solution that uses unmanaged code, try
Shut Down, Restart, Log off or Lock your computer in C#[^]