Q161301: HOWTO: Pass a Point Structure to the Windows API by Value

Article: Q161301
Product(s): Microsoft Visual Basic for Windows
Version(s): 
Operating System(s): 
Keyword(s): kbGrpDSVB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SUMMARY
=======

The WindowFromPoint and ChildWindowFromPoint API functions require that a Point
structure be passed by value. Visual Basic is not capable of passing a structure
by value, only by reference. However, it is possible to pass the elements of the
user-defined type in the correct order so that these API functions can be used.
This article explains and demonstrates how to pass the Point structure by value
to the API.

MORE INFORMATION
================

To preserve compatibility with previous Windows APIs, Microsoft decided that the
Point structure should continue to be passed by value for the WindowFromPoint
and ChildWindowFromPoint API functions in the 32-bit user dynamic link library
(DLL).

Visual Basic is not capable of passing structures by value, but the value of the
elements of the structure can be placed on the stack in the correct order.
Therefore, when passing the elements of the Point structure, two long values
should be passed.

Another important consideration is that Visual Basic uses the C convention
(stdcall) of passing parameters. This convention specifies that arguments are
placed on the stack from right to left. API functions are declared using the
Pascal calling convention where the elements of the structure must be listed in
reverse order (that is, element y followed by x) when calling the
WindowFromPoint function using 32-Bit Visual Basic.

Step-by-Step Example
--------------------

1. Start Visual Basic 5.0 and select Standard EXE. If it is already running,
  click New Project on the File menu and select Standard EXE. Form1 is created
  by default.

2. Change the ScaleMode of the form to 3 - Pixel. (The coordinates of the Point
  must be passed in pixels.)

3. Insert a new code module in the project (on the Project menu, click Add
  Module). Place the following code in the new module:

        Declare Function WindowFromPointXY Lib "User32" _
            Alias "WindowFromPoint" (ByVal xPoint As Long, _
            ByVal yPoint As Long) As Long

        Function VBWindowFromPoint(ByVal x As Long, ByVal y As Long) As Long
            VBWindowFromPoint = WindowFromPointXY(x, y)
        End Function

4. Place the following code in the MouseDown event of Form1:

        ' Convert form coordinates to screen coordinates
        screenX = X + Form1.Left / Screen.TwipsPerPixelX
        screenY = Y + Form1.Top / Screen.TwipsPerPixelY

        ' Retrieve the window handle
        hWindow = VBWindowFromPoint(screenX, screenY)

        If hWindow = Form1.hWnd Then
        MsgBox "Success"
        End If

5. Run the code by pressing the F5 key. Click Form1. The message box should
  appear indicating that the API call successfully returned the window handle
  of Form1.

Additional query words: kbVBp500 kbVBp600 kbVBp kbdsd kbDSupport KBWIN32SDK KBAPI

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Keywords          : kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Issue type        : kbhowto

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