AddressOf Operator

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Named Arguments

No

Syntax

AddressOf procedurename

procedurename

Use: Required

The name of an API procedure.

Description

Passes the address of a procedure to an API function. There are some API functions that require the address of a callback function as a parameter. (A callback function is a routine in your code that is invoked by the routine that your program is calling: it calls back into your code.) These callback functions are passed to the API function as pointers to a memory address. In the past, calling functions that required callbacks posed a unique problem to VB, since, unlike C or C++, it lacks a concept of pointers. However, the AddressOf operator allows you to pass such a pointer in the form of a long integer to the API function, thereby allowing the API function to call back to the procedure.

Rules at a Glance

• The callback function must be stored in a code module; attempting to store it in a class or a form module generates a compile-time error, "Invalid use of AddressOf operator."

• The AddressOf operator must be followed by the name of a user-defined function, procedure, or property.

• The data type of the corresponding argument in the API function's Declare statement must be As Any or As Long.

• The AddressOf operator can't call one VB procedure from another.

Example

The following example uses the EnumWindows and GetWindowText API calls to return a list of currently open windows. EnumWindows requires the address of a callback function as its first parameter. A custom function, EnumCallBackProc, is the callback function that populates the lstWindowTitles list box.

When the cmdListWindows command button is clicked, the list box is cleared, and a call to the EnumWindows API function is made, passing the AddressOf the EnumCallBackProc function and a reference to the list box control. EnumWindows then calls back to EnumCallBackProc, passing it the window handle of an open window and the reference to the list box. EnumCallBackProc then uses the GetWindowText API function to return the text in the titlebar of the window, passing it the window handle, a string buffer, and the length of that buffer. EnumCallBackProc is called by the API function as many times as is required, depending upon the number of open windows. The first portion of the example code must be stored in a code module, while the cmdListWindows_Click event handler can be stored in the form module containing the cmdListWindows button.

Option Explicit

Public Declare Function EnumWindows Lib "User32" _
                             (ByVal lpEnumFunc As Any, _
                             ByVal lParam As Any) As Long

Public Declare Function GetWindowText Lib "User32" _
                                Alias "GetWindowTextA" _
                                (ByVal hWnd As Long, _
                                 ByVal lpString As String, _
                                 ByVal cch As Long) As Long

Function EnumCallBackProc(ByVal hWnd As Long, _
                          ByVal lParam As ListBox) As Long

    On Error Resume Next

    Dim sWindowTitle As String
    Dim lReturn As Long

    sWindowTitle = String(512, 0)

    lReturn = GetWindowText(hWnd, sWindowTitle, 512)

    If lReturn > 0 Then
        lParam.AddItem sWindowTitle
    End If

    EnumCallBackProc = 1

End Function

Private Sub cmdListWindows_Click()

   Dim lReturn As Long
   
   lstWindowTitles.Clear
   lReturn = EnumWindows(AddressOf EnumCallBackProc, _
                         lstWindowTitles)

End Sub

Programming Tips and Gotchas

• Debugging calls containing AddressOf is at best very difficult and most of the time downright impossible.

• It's possible to pass an AddressOf pointer from one VB procedure to another by creating a wrapper for the callback function. To do this, however, you must declare the pointer as either Long or Any. The following snippet shows how you could add such a wrapper function to the example used above:

  Private Sub cmdListWindows_Click()
  
  Dim lReturn As Long
  
  lReturn = DoWindowTitles(AddressOf EnumCallBackProc, _
                           lstWindowTitles)
  End Sub
  
  Private Function DoWindowTitles(CallBackAddr As Long, _
                            lstBox As ListBox) As Long
  
      'other stuff here
      lstBox.Clear
      DoWindowTitles = EnumWindows(CallBackAddr, lstBox)
          
  End Function

  
  

• Because you can't pass an error back to the calling Windows API function from within your VB callback function, you should use the On Error Resume Next statement at the start of your VB callback function.

See Also

Declare Statement