How to unlock windows programmatically
No, this is not possible. Is that definitive enough for you?
Whether you're concerned with security or not, the Windows team is and absolutely must be. Consequently, there is no API provided to unlock a secured desktop.
It wouldn't make sense anyway—you have to enter a password in order to unlock the workstation, and how is a function call going to enter a password?
UI automation or whatever some ingenious hacker might cook up isn't going to work because the secure attention sequence (Ctrl+Alt+Del) is required to prevent people from doing precisely this.
The only solution would be to completely replace the Windows security system. On Windows XP, you'd do that by writing a new GINA DLL to replace the standard msgina.dll. Things got slightly more complicated in Windows Vista, where GINA was replaced with the new Credential Providers. You might find this article useful if you're interested in pursuing such a route.
How to unlock windows 8/8.1/10 from another PC programmatically?
You can write your own Credential Provider library (C++ only) and control it remotely somehow. Or use LogonExpert remotely (via PsExec or your own means).
Program to enter password and unlock windows 7, 8, 10
You can't. This is one of those things explicitly forbidden for applications to do. The 'lock' activates the Secure Desktop (see What happens 'behind' the windows lock screen?, although note that the Secure Desktop moniker is overloaded). This mode is designed to prevent being deactivated without user explicitly providing it's credentials. Any 'circumvention' would be, by definition, a bug. For a more detailed explanation, read the topic of Winlogon and GINA.
The linked Steam article is a horrid hack, it simply deactivates the timeout locking, overriding the user chosen setting, it has nothing to do with actually exiting from the 'lock' mode.
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