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The LastErr.vbp project (on the CD that comes with this book) has a form with a text box named txtCaption into which you can type the caption of the main window of any program. The FindWindow function retrieves the handle to that window. The program then uses the PostMessage function to post a WM_CLOSE message into the message queue for the windowthis is the same message that is posted when you use the close box or system menu to close a window. |
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Private Declare Function PostMessage Lib "User32" Alias _
"PostMessageA" (ByVal hWnd As Long, ByVal Message As Long, ByVal _
wParam As Long, ByVal lParam As Long) As Long
Private Const WM CLOSE = &H10
Private Sub cmdClose_Click()
Dim WindowHandle As Long
WindowHandle = FindWindow("", txtCaption.Text)
If WindowHandle = 0 Then
MsgBox "No window: Last Error was " & Err.LastDllError
Exit Sub
Else
Call PostMessage(WindowHandle, WM_CLOSE, 0, 0)
End If
End Sub |
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The default caption is ''Untitled Notepad," which is the caption of a default Notepad application window. The program fails. The message box displays the LastError code, a value provided by the operating system that explains the reason for the error.2 Your mission is twofold: |
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1. Find the meaning of the last error code. |
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2. Fix the program so that it will work. |
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2 The LastDllError code will be 123 on Windows NT, 0 on Windows 95/98. |
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