Click here to Skip to main content
15,219,906 members
Rate this:
Please Sign up or sign in to vote.
See more:
Hi everyone,

First of all I want to thank everyone in advance and to apologize because I have very little experience playing with wndproc and messages.

I'm using the code of the following link (converted to Visual Basic and a bit modified) to intercept the keys from a barcode scanner and pass them to the instantiating class when the enter key is received, not letting the keydown be processed using prefiltermessage to don't let the key be processed in the focuseed control/form.

Distinguishing Barcode Scanners from the Keyboard in WinForms

At the moment I don't have a barcode scanner, but for testing purposes, I'm using a second USB keyboard attached to the computer, that should do the trick.

The following subs/functions are the basic core of the barcode scanner intercepter. There is a C++ interop library developed by the author of the previous link for making the GetRawInputInfo and handling the devices, although I think they have nothing to do with the problem.
Public Class BarcodeScannerListener
    Private keystrokeBuffer As StringBuilder
    Friend Event BarcodeScanned(BarCode As String)
    Protected Overrides Sub WndProc(ByRef m As Message)
        Select Case m.Msg
            Case WM_INPUT
                Dim deviceInfo As BarcodeScannerDeviceInfo = Nothing
                Dim handled As Boolean = False
                Dim keystroke As Boolean = False
                Dim localBuffer As String = String.Empty
                Dim rawInputDeviceHandle As IntPtr = IntPtr.Zero

                Me.interopHelper.GetRawInputInfo(m.LParam, rawInputDeviceHandle, keystroke, localBuffer)

                If Me.devices.TryGetValue(rawInputDeviceHandle, deviceInfo) AndAlso keystroke Then

                    Me.filter.FilterNext = True
                    If localBuffer.Length = 1 AndAlso Asc(localBuffer(0)) = 13 Then
                        Dim barcode As String = Me.keystrokeBuffer.ToString()

                        If barcode IsNot Nothing AndAlso barcode.Length > 0 Then
                            Me.keystrokeBuffer = New StringBuilder()
                            RaiseEvent BarcodeScanned(barcode)
                        End If
                    End If
                    Me.filter.FilterNext = False
                End If
                Exit Select
        End Select
    End Sub
End Class

Public Class BarcodeScannerKeyDownMessageFilter
    Implements IMessageFilter
    Public Property FilterNext() As Boolean
    Public Function PreFilterMessage(ByRef m As Message) As Boolean Implements System.Windows.Forms.IMessageFilter.PreFilterMessage
        Dim filter As Boolean = False
        If Me.FilterNext AndAlso m.Msg = WM_KEYDOWN Then
            filter = True
            Me.FilterNext = False
        End If
        Return filter
    End Function
End Class

Class UsingBarcode
    Private WithEvents CodeReaderHandler As New BarcodeScannerListener
    Private Sub OnBarcodeScanned(Barcode As String) Handles CodeReaderHandler.BarcodeScanned
    End Sub
End Class

After receiving the Enter key from the keyboard the barcode string is perfectly sent using the BarcodeScanned event, but the msgbox is not shown, but I can hear the bip os the messagebox in the speakers. If I put a second msgbox just after the first one, the second msgbox works perfectly.

It looks like some message from the Enter key is passing through to the msgbox and the default button is being clicked before the user does, so the first msgbox dissapears and the second works fine. For what I can see, a keydown event is never fired for the Enter key, and a keyup event is being fired and it gets to the instantiating form. I've tried to put an stop in the prefiltermessage with the keyup message and setting the filter to true manually after pressing the Enter key, but still the key gets to the form's keyup event, so I supossed is getting to the messasgebox too.

As a workaround I'm firing 2 msgbox instead of one, but is not an elegant solution.

Anyone knows the answer? I've been trying to look for a solution for myself and in Google, but I couldn't find any, so any help will be really appreciated.
nilesh sawardekar 20-Jul-14 16:03pm
and summerized story is that.. I am looking for what?.. please explain in short and sweet..
I am working on barcode technology from last 6 i can solve your problem..
damuvi 20-Jul-14 16:11pm
You are right, after all this long explanation, I really didn't sumarize the problem.
In only a few words, a msgbox in an event procedure (OnBarcodeScanned) launched from the wndproc dissapears without the user intervention (as the matter of fact I don't even get to see it), but if I launch a second msgbox, this is shown correctly.
I expect this time is a bit more clear.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

CodeProject, 503-250 Ferrand Drive Toronto Ontario, M3C 3G8 Canada +1 416-849-8900 x 100