DOCUMENT:Q187918  20-JUN-1998  [vbwin]
TITLE   :PRB: SendMessage Fails for Some Messages
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:5.0
OPER/SYS:WINDOWS
KEYWORDS:

======================================================================
---------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Visual Basic Learning, Professional, and Enterprise Editions
   for Windows, version 5.0
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SYMPTOMS
========

The Sendmessage API function call will fail to give expected results for
some messages when the API Text Viewer declaration is used. For example,
the messages "WM_SETTEXT" and "LVM_SCROLL" will fail without returning any
error values.

CAUSE
=====

Some messages expect the lParam to be passed as ByVal, while the API Text
Viewer declaration passes lParam as ByRef.

RESOLUTION
==========

The declaration can be manually changed after copying the declaration.

STATUS
======

This behavior is by design.

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

When a message expects the lParam as string, then the Sendmessage
declaration must be changed to pass lParam as ByVal. Additionally, some
messages require integer values to be passed as ByVal. Because the
requirements for messages vary, it is necessary to check the documentation
for each message for details.

Steps to Reproduce Behavior
---------------------------

1. Start a new project. Form1 is created by default.

2. Add a CommandButton and a TextBox to Form1.

3. Paste the following code in the General Declaration section of Form1:

      Option Explicit
      Private Declare Function SendMessage Lib "user32" Alias _
         "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, _
         ByVal wParam As Long, lParam As Any) As Long
      Const WM_SETTEXT = &HC
      Private lRet As Long

      Private Sub Command1_Click()
          lRet = SendMessage(Text1.hwnd, WM_SETTEXT, 0, "hello")
      End Sub

4. Run the project and click Command1. Some garbage is placed in the
   textbox. However, SendMessage returns 1 indicating that SendMessage has
   succeeded.

5. Change the declaration of SendMessage so that the lParam argument is
   passed ByVal:

      Private Declare Function SendMessage Lib "user32" Alias _
         "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, _
         ByVal wParam As Long, ByVal lParam As Any) As Long

6. Run the project once more and click Command1. Note that Text1 now
   contains the text "hello" as expected.

Additional query words: kbDSupport kbDSD kbVBp500 kbVBp kbAPI kbSDKWin32
======================================================================
Version           : WINDOWS:5.0
Platform          : WINDOWS
Issue type        : kbprb

=============================================================================

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