Question

Firstly, wikipedia defines a Control Panel Applet as:

graphical user interface which allows users to view and manipulate
basic system settings and controls via applets

Is this correct?

Secondly, what is the difference between a VCL application and a Control Panel applet in terms of functionality. Is it easier to change basic system settings using a CP Applet compared to a VCL application?

Answer 1

A control panel applet is a DLL that meets certain criteria to display within the Windows Control Panel. (See the MSDN documentation for CPlApplet() for details.) So the answer to your first question is no, that definition is not correct, and the answer to the second is: A VCL Forms application is a standard Windows application based on the Delphi VCL, and a Control Panel Applet is a DLL that is designed to be loaded into the Windows Control Panel application (you’ll find it with Start->Control Panel on most Windows versions since Win95).

According to this CodeProject article:

A control panel applet is a DLL that presents a standard interface to the Control Panel application. When Control Panel starts it locates all the .cpl files in the Windows System directory and tries to load them as DLLs. If this works then it tries to locate a function called CPlApplet() in the newly loaded DLL. If the DLL has the function entry point then it’s a control panel applet! But there’s more, a single DLL can contain multiple applets and they each appear as a separate icon in Control Panel.

CPlApplet() is a relatively simple function taking a window handle, a message and a couple of message specific parameters. Writing one is similar to writing a normal window proc and is reasonably well documented in MSDN and the VC++ online help.