How To Pass a Point Structure to the Windows API by Value

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.