PRB: SendMessage Fails for Some Messages

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

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 kbVBp600

Keywords : kbGrpVB
Version :
Platform : WINDOWS
Issue type : kbprb