AddressOf Operator
A unary operator that causes the address of the procedure it precedes to be passed to an API procedure that expects a function pointer at that position in the argument list.
Syntax
AddressOf procedurename
The required procedurename specifies the procedure whose address is to be passed. It must represent a procedure in a standard module module in the project in which the call is made.
Remarks
When a procedure name appears in an argument list, usually the procedure is evaluated, and the address of the procedure's return value is passed. AddressOf permits the address of the procedure to be passed to a Windows API function in a dynamic-link library (DLL), rather passing the procedure's return value. The API function can then use the address to call the Basic procedure, a process known as a callback. The AddressOf operator appears only in the call to the API procedure.
Although you can use AddressOf to pass procedure pointers among Basic procedures, you can't call a function through such a pointer from within Basic. This means, for example, that a class written in Basic can't make a callback to its controller using such a pointer. When using AddressOf to pass a procedure pointer among procedures within Basic, the parameter of the called procedure must be typed As Long.
Warning Using AddressOf may cause unpredictable results if you don't completely understand the concept of function callbacks. You must understand how the Basic portion of the callback works, and also the code of the DLL into which you are passing your function address. Debugging such interactions is difficult since the program runs in the same process as the development environment. In some cases, systematic debugging may not be possible.
 Note
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| You can create your own call-back function prototypes in DLLs compiled with Microsoft Visual C++ (or similar tools). To work with AddressOf, your prototype must use the __stdcall calling convention. The default calling convention (__cdecl) will not work with AddressOf. |
Since the caller of a callback is not within your program, it is important that an error in the callback procedure not be propagated back to the caller. You can accomplish this by placing the On Error Resume Next statement at the beginning of the callback procedure.
Example
The following example creates a form with a list box containing an alphabetically sorted list of the fonts in your system.
To run this example, create a form with a list box on it. The code for the form is as follows:
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Place the following code in a module. The third argument in the definition of the EnumFontFamilies function is a Long that represents a procedure. The argument must contain the address of the procedure, rather than the value that the procedure returns. In the call to EnumFontFamilies, the third argument requires the AddressOf operator to return the address of the EnumFontFamProc procedure, which is the name of the callback procedure you supply when calling the Windows API function, EnumFontFamilies. Windows calls EnumFontFamProc once for each of the font families on the system when you pass AddressOf EnumFontFamProc to EnumFontFamilies. The last argument passed to EnumFontFamilies specifies the list box in which the information is displayed.
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