BUG: WM_LBUTTONUP Message May Not Fire

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

SYMPTOMS

If the validate event of a TextBox displays a MessageBox, shifting the focus to another control using the mouse causes the MessageBox to display. It also causes the WM_LBUTTONUP message for the control receiving the focus to be lost.

For example, if the control that is receiving the focus is a CommandButton, the Click event is not fired. If the control that is receiving the focus is a TextBox, notice that it appears to be in the selection state as if the left-mouse button is pressed.

CAUSE

When the message box is displayed, the focus moves to the message box, so the control that gets the focus does not get the WM_LBUTTONUP message.

RESOLUTION

To solve the problem, you can send a WM_LBUTTONUP message programmatically to the control that gets the focus.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

Steps to Reproduce Behavior

1. Create a Standard EXE project. Form1 is created by default.



2. Add two TextBoxes and one CommandButton to Form1.



3. Add the following code to the code window of Form1:




Option Explicit

Private Sub Text1_Validate(Cancel As Boolean)
    MsgBox "Text1 Validate Message"
End Sub

Private Sub Text2_MouseDown(Button As Integer, Shift As Integer, _
                            X As Single, Y As Single)
    Debug.Print "Text2 Mouse Down"
End Sub

Private Sub Text2_MouseUp(Button As Integer, Shift As Integer, _
                          X As Single, Y As Single)
    Debug.Print "Text2 Mouse Up"
End Sub

Private Sub Command1_MouseDown(Button As Integer, Shift As Integer, _
                               X As Single, Y As Single)
    Debug.Print "Command1 Mouse Down"
End Sub

Private Sub Command1_MouseUp(Button As Integer, Shift As Integer, _
                             X As Single, Y As Single)
    Debug.Print "Command1 Mouse Up"
End Sub

Private Sub Command1_Click()
    MsgBox "Command1 Click"
End Sub 

4. Run the application. Move the focus to Text1 and then either select Text2 or click Command1. Observe the debug output in the immediate window and notice that the MouseUp event does not fire. You can also observe the other behavior described in the SYMPTOMS section.

Steps to Work Around the Behavior

1. Building on the sample above, add the following code to the code window of Form1:




Private Const WM_LBUTTONUP = &H202

Private Declare Function PostMessage Lib "user32" _
    Alias "PostMessageA" _
    (ByVal hwnd As Long, ByVal wMsg As Long, _
    ByVal wParam As Long, ByVal lParam As Long) As Long

Private Sub ForceMouseUp()
    Dim ctl As Control
    For Each ctl In Me.Controls
        PostMessage ctl.hwnd, WM_LBUTTONUP, 0, ByVal 0
    Next
End Sub 

2. Change the Text1_Validate event handler to be:




Private Sub Text1_Validate(Cancel As Boolean)
    MsgBox "Text1 Validate Message"
    ForceMouseUp
End Sub 

3. Run the application and you can see the correct behavior.


NOTE: You only need to call the ForceMouseUp when you do not want to set Cancel to True.

Additional query words:

Keywords : kbVBp600bug kbWndwMsg kbGrpVB kbDSupport
Version : WINDOWS:6.0
Platform : WINDOWS
Issue type : kbbug