Why do some program after installation need the system to be restarted?

Question

I was asked this question in an interview.

After installation of programs on windows OS,some program need the system to be restarted while others not.

What is the reason behind this?

I googled but didn't get satisfactory answer.

Thanks in advance.

I'm not an expert but I would think it would be files that are loaded on boot or that are continusly in use as you cant replase a file that is being used — Qwertie, Apr 19 '14 at 06:28

score 1 · Answer 1 · edited May 23 '17 at 12:00

If a program (or software) need restart, it is because of one of the following conditions is true :

The software updates a DLL that is loaded in one or more processes that are required by Windows. The update cannot be completed while the DLL is loaded. Therefore, the software update must stop the process that causes the DLL to be loaded. Stopping the process will unload the DLL that is required to complete the update. However, the process in which the DLL is loaded cannot be stopped while Windows is running.
The software updates an .exe file that is currently running as a process that is required by Windows. The update cannot be completed while this process is running. However, you cannot force this process to stop unless you shut down Windows.
The software updates a device driver that is currently being used and that is required by Windows. The update cannot be completed while this device driver is being used. However, you cannot unload this device driver unless you shut down Windows.
The software makes changes to the registry. These changes require that you restart your computer.

Other references

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