Subclassing and Hooking with Visual Basic

The AddressOf Operator

The AddressOf operator, first introduced in Version 5 of VB, gave developers limited access to pointers, a feature that VB effectively hides from them but that is essential to high-end development environments such as Visual C++. The AddressOf operator greatly increases the potential of a VB application. As we shall see, though, there are always bumps in the road when implementing more advanced functionality, and AddressOf has several of them.

AddressOf provides the VB developer with a simple way of using function pointers without relying on another language. A function pointer is simply a variable that contains the memory location of a single function. In other words, this variable points to a function. Now, instead of having to use the function name to call the function, we can instead use the function pointer to call the function.

A callback or callback function is the function which the function pointer references. Code that receives a function pointer can use it to call back (hence the name “callback”) to that function. Usually, these callback functions are small in size because they might be called many times per second and affect application performance.

Function pointers and callback functions are mainly used for asynchronous processing and with the enumeration application programming interface (API) functions. EnumWindows, EnumChildWindows, and EnumDesktopWindows are just some of Windows’ enumeration API functions. These functions each take ...