How To Package HiWord/LoWord Values Into a Long Parameter

This article was previously published under Q189170
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For some API functions, such as SendMessage or PostMessage, you may need topackage two short Integer values into a Long variable to pass them as asingle parameter. This article demonstrates how to package such Integersand how to unpack them if necessary.
The trick to packing values is bit shifting. Because Visual Basic does notprovide bit shift operators to use, you need to do things the old fashionedway; through multiplication. To make an Integer the high word for a Longvalue, you need to multiply it by &H10000. This has the effect of shiftingthe bit values 16-bits (2-bytes) to the left, making room for the low wordvalue you want to add.

Before you can add the low word value, however, you need to make anadjustment. Remember that Visual Basic Integer types are signed values, butthe low word value needs to be unsigned if you plan to add it to your highword value. To make sure Visual Basic treats the low word as an unsignedinteger, you need to perform a bitwise "And" on the value using &HFFFF& asa mask. In effect, this saves the value as a Long integer with the high(signed) bit cleared but keeps the original Integer's bit value preserved.

With this in mind, here is a Visual Basic function that creates a Longvalue from two Integers:
   Function MakeDWord(LoWord As Integer, HiWord As Integer) As Long      MakeDWord = (HiWord * &H10000) Or (LoWord And &HFFFF&)   End Function				
Only a few API functions require that you unpack a double word into twoshort Integers. If you find that you need to perform such a task, you canuse conversion routines similar to these:
   Function LoWord(DWord As Long) As Integer      If DWord And &H8000& Then ' &H8000& = &H00008000         LoWord = DWord Or &HFFFF0000      Else         LoWord = DWord And &HFFFF&      End If   End Function   Function HiWord(DWord As Long) As Integer      HiWord = (DWord And &HFFFF0000) \ &H10000   End Function				
Both these functions simply mask the portion of the Long integer they wantto return using the bitwise "And" operator. The HiWord function shifts thisvalue right 16-bits by dividing it by &H10000. The LoWord function cansimply return the value of the "And" operation in most cases. However,because Visual Basic Integers are signed, any low word value that has itshigh bit set must be converted back into a negative value using the "Or"operator and a mask of &HFFFF0000.

Sample Code

  1. Open a new Standard EXE project in Visual Basic. Form1 is created by default.
  2. Add a CommandButton to the lower-right corner of Form1.
  3. Add the following code to the General Declaration section for Form1:
          Private Declare Function SendMessage Lib "user32" _         Alias "SendMessageA" (ByVal hwnd As Long, _                               ByVal wMsg As Long, _                               ByVal wParam As Long, _                               ByVal lParam As Long) As Long      Private Const MK_LBUTTON = &H1      Private Const WM_LBUTTONDOWN = &H201      Function MakeDWord(LoWord As Integer, HiWord As Integer) As Long         MakeDWord = (HiWord * &H10000) Or (LoWord And &HFFFF&)      End Function      Private Sub Form_MouseDown(Button As Integer, Shift As Integer, _      X As Single, Y As Single)         Form1.Cls         Form1.Print "Button Click Event Fired"         Form1.Print "Position X:" & Str$(X / Screen.TwipsPerPixelX)         Form1.Print "Position Y:" & Str$(Y / Screen.TwipsPerPixelY)      End Sub      Private Sub Command1_Click()         Dim nMousePosition As Long         ' nMousePosition stores the x (hiword) and y (loword) values         ' of the mouse cursor as measured in pixels.         Let nMousePosition = MakeDWord(16, 18)         Call SendMessage(Me.hwnd, WM_LBUTTONDOWN, MK_LBUTTON, _         nMousePosition)      End Sub						
  4. Press the F5 key to run the project.

    You will notice that anywhere you click on the Form, the values of the mouse cursor are displayed (in pixels). When you click the CommandButton, the SendMessage call simulates a mouse click at the location 16 pixels right of the left side of the Form and 18 pixels down from the top, causing the Form_MouseDown event to fire.
For additional information, please see the following article in theMicrosoft Knowledge Base:
112651 How To Mimic HIWORD, LOWORD, HIBYTE, LOBYTE C Macros in VB
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Article ID: 189170 - Last Review: 02/24/2014 08:33:35 - Revision: 3.1

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