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.
The required procedurename specifies the procedure whose address is to be passed. It must represent a procedure in a standard module in the project in which the call is made.
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 than 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 by 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.
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 because the program runs in the same process as the development environment. In some cases, systematic debugging may not be possible.
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.
Because 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.
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:
Option Explicit Private Sub Form_Load() Module1.FillListWithFonts List1 End Sub
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.
'Font enumeration types Public Const LF_FACESIZE = 32 Public Const LF_FULLFACESIZE = 64 Type LOGFONT lfHeight As Long lfWidth As Long lfEscapement As Long lfOrientation As Long lfWeight As Long lfItalic As Byte lfUnderline As Byte lfStrikeOut As Byte lfCharSet As Byte lfOutPrecision As Byte lfClipPrecision As Byte lfQuality As Byte lfPitchAndFamily As Byte lfFaceName(LF_FACESIZE) As Byte End Type Type NEWTEXTMETRIC tmHeight As Long tmAscent As Long tmDescent As Long tmInternalLeading As Long tmExternalLeading As Long tmAveCharWidth As Long tmMaxCharWidth As Long tmWeight As Long tmOverhang As Long tmDigitizedAspectX As Long tmDigitizedAspectY As Long tmFirstChar As Byte tmLastChar As Byte tmDefaultChar As Byte tmBreakChar As Byte tmItalic As Byte tmUnderlined As Byte tmStruckOut As Byte tmPitchAndFamily As Byte tmCharSet As Byte ntmFlags As Long ntmSizeEM As Long ntmCellHeight As Long ntmAveWidth As Long End Type ' ntmFlags field flags Public Const NTM_REGULAR = &H40& Public Const NTM_BOLD = &H20& Public Const NTM_ITALIC = &H1& ' tmPitchAndFamily flags Public Const TMPF_FIXED_PITCH = &H1 Public Const TMPF_VECTOR = &H2 Public Const TMPF_DEVICE = &H8 Public Const TMPF_TRUETYPE = &H4 Public Const ELF_VERSION = 0 Public Const ELF_CULTURE_LATIN = 0 ' EnumFonts Masks Public Const RASTER_FONTTYPE = &H1 Public Const DEVICE_FONTTYPE = &H2 Public Const TRUETYPE_FONTTYPE = &H4 Declare Function EnumFontFamilies Lib "gdi32" Alias _ "EnumFontFamiliesA" _ (ByVal hDC As Long, ByVal lpszFamily As String, _ ByVal lpEnumFontFamProc As Long, LParam As Any) As Long Declare Function GetDC Lib "user32" (ByVal hWnd As Long) As Long Declare Function ReleaseDC Lib "user32" (ByVal hWnd As Long, _ ByVal hDC As Long) As Long Function EnumFontFamProc(lpNLF As LOGFONT, lpNTM As NEWTEXTMETRIC, _ ByVal FontType As Long, LParam As ListBox) As Long Dim FaceName As String Dim FullName As String FaceName = StrConv(lpNLF.lfFaceName, vbUnicode) LParam.AddItem Left$(FaceName, InStr(FaceName, vbNullChar) - 1) EnumFontFamProc = 1 End Function Sub FillListWithFonts(LB As ListBox) Dim hDC As Long LB.Clear hDC = GetDC(LB.hWnd) EnumFontFamilies hDC, vbNullString, AddressOf EnumFontFamProc, LB ReleaseDC LB.hWnd, hDC End Sub