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

MORE INFORMATION

The Windows 16-bit API is unusal because it allows you to pass the Point structure to the WindowFromPoint functions by value. Typically, user- defined types are passed to functions by reference because passing them by value can use up a lot of stack space and be much slower than passing the address of the structure. An exception was made for the Point structure in certain functions (such as ChildWindowFromPoint and WindowFromPoint) because in 16-bit programming the size of the structure was only 32 bits long, which is the same length as a far address anyway.

However, on a 32-bit operating system, this structure becomes 64 bits. To preserve compatibility, Microsoft decided that the Point structure should still be passed by value for these two 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 in 32-bit programming, two long values should be passed. When passing the elements in 16-bit programming, two integers should be passed to the function.

Another important consideration is that 32-bit Visual Basic uses the C convention (stdcall) of passing parameters. This convention specifies that arguments are placed on the stack from right to left. 16-Bit Visual Basic maintains the Pascal convention of passing parameters from left to right. (API functions are declared using the Pascal calling convention.) As a result, 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. When using 16-bit Visual Basic, element x is passed to the API function before element y.

Step-by-Step Example of How to Call the WindowFromPoint API

The code in the following example uses conditional compilation so that the code can be placed in either the 16-bit or 32-bit version of Visual Basic.

1. Start a new project in Visual Basic. Form1 is created by default.
2. Change the ScaleMode of the form to 3 - Pixels. (The coordinates of the Point must be passed in pixels.)
3. Insert a new code module in the project (on the Insert menu, click New Module). Place the following code in the new module:

      #If Win32 Then
      Declare Function WindowFromPointXY Lib "User32" _
          Alias "WindowFromPoint" (ByVal xPoint As Long, _
          ByVal yPoint As Long) As Long
      #Else
      Declare Function WindowFromPointYX Lib "User" _
          Alias "WindowFromPoint" (ByVal yPoint As Integer, ByVal _
          xPoint As Integer) As Integer
      #End If
   
      Function VBWindowFromPoint(ByVal x As Long, ByVal y As Long) As Long
      #If Win32 Then
          VBWindowFromPoint = WindowFromPointXY(x, y)
      #Else
          VBWindowFromPoint = WindowFromPointYX(y, x)
      #End If
      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.