Problems with barcode reading using PreFilterMessage

Question

I have a task of reading a barcode from a Symbol LS2208 barcode scanner.
The scanner is setup deafult but with a prefix of F13 and surfix of 'Enter'. Using this the scanner emulates a US keyboard. My keyboard is Danish and OS language is set to Danish.
I like to be independend of users regional settings.

Right now I am implementing the IMessageFilter using:

    private const int WM_KEYDOWN = 0x100;
    private List<Keys> keysSequence = new List<Keys>();
    public bool PreFilterMessage(ref Message m)
    {
        if (m.Msg == WM_KEYDOWN)
        {
            Keys keyCode = (Keys)(int)m.WParam & Keys.KeyCode;
            if (keyCode == Keys.F13)
            {
                ...
                return true;
            }
            if (keyCode == Keys.Enter)
            {
                ...
                string barcode = GenerateBarcode(keysSequence);
                return true;
            }

            keysSequence.Add(keyCode);
        }
    }

When the surfix 'Enter' key is received sequence of keys are used to create the barcode string with the method GenerateBarcode:

    public static string GenerateBarcode(Keys[] captureKeysSequence)
    {
        StringBuilder barcodeBuffer = new StringBuilder();
        bool shift = false;
        bool altGr = false;
        foreach (Keys keyCode in captureKeysSequence)
        {
            if (keyCode == Keys.Shift || keyCode == Keys.ShiftKey)
            {
                shift = true;
                continue;
            }
            if (keyCode == (Keys.Control | Keys.Alt))
            {
                altGr = true;
                continue;
            }
            barcodeBuffer.Append(GetCharsFromKeys(keyCode, shift, altGr));
            shift = false;
            altGr = false;
        }
        return barcodeBuffer.ToString();
    }

I am handling the shift and altGr key here.

Now comes the cause of my problems the GetCharsFromKeys method:

    [DllImport("user32.dll")]
    public static extern int ToUnicodeEx(uint virtualKeyCode, uint scanCode,
            byte[] keyboardState,
            [Out, MarshalAs(UnmanagedType.LPWStr, SizeConst = 64)]
            StringBuilder receivingBuffer,
            int bufferSize, uint flags, IntPtr dwhkl);

    public static string GetCharsFromKeys(Keys keys, bool shift, bool altGr)
    {
        var buf = new StringBuilder(256);
        var keyboardState = new byte[256];
        if (shift)
            keyboardState[(int)Keys.ShiftKey] = 0xff;
        if (altGr)
        {
            keyboardState[(int)Keys.ControlKey] = 0xff;
            keyboardState[(int)Keys.Menu] = 0xff;
        }

        // Because the Symbol LS2208 maps a US keyboard we need to convert the Keys correctly to chars
        ToUnicodeEx((uint)keys, 0, keyboardState, buf, buf.Capacity, 0, InputLanguage.FromCulture(new System.Globalization.CultureInfo("en-US")).Handle);

        return buf.ToString();
    }

I am trying to convert the Keys input from the barcode scanner to a string. And for most of the common chars this is working perfetly (both lower and upper characters and numbers) and also some of the special chars found above the numbers on the keyboard is working (eg. '$').

But when testing with a barcode like "12 / 34 - 56" I end up with the output "12 - 34 = 56"?

I think it have something to do with the mapping between US and Danish keyboard but I cannot figure out why?
Can anybody help me with this conversion?

Answer 1

I haven't be able to solve this problem the way I wanted to begin with:
"By using a Barcode scanner setup to use US keyboard, and receiving the characters independend of the set Windows region keyboard settings."

So I had to limit my requirements to:
"The Barcode scanner and Windows must have the same keyboard settings."

This makes everything so much simpler. I still implementing the IMessageFilter and use both VM_KEYDOWN and VM_MSG. Here is the code I use (though, still lacking some error/timeout handling):

public class BarcodeScannerMessageFilter : IMessageFilter
{
    public event EventHandler<BarcodeScannerReadyEventArgs> BarcodeScannerReady;

    private bool barcodeStarted = false;
    private StringBuilder barcodeBuilder = new StringBuilder();

    public BarcodeScannerMessageFilter()
    {
        Application.AddMessageFilter(this);
    }

    #region IMessageFilter Members
    private const int WM_KEYDOWN = 0x100;
    private const int WM_MSG = 0x102;
    public bool PreFilterMessage(ref Message m)
    {
        if (m.Msg == WM_KEYDOWN) // Use KeyDown to look for prefix and surfix
        {
            Keys keyCode = (Keys)(int)m.WParam & Keys.KeyCode;

            if (!barcodeStarted) // Prevent F13 Capture until previous is ended
            {
                // Check for start capture key (F13)
                if (keyCode == Keys.F13)
                {
                    barcodeStarted = true;
                    return true;
                }
            }
            else
            {
                // Check for end capture key (Enter)
                if (keyCode == Keys.Enter)
                {
                    // Raise barcode capture event with barcode
                    RaiseBarcodeReadyEventAsync(barcodeBuilder.ToString());

                    // End sequence
                    barcodeBuilder.Clear();
                    barcodeStarted = false;
                    return true;
                }
            }
        }
        else if (m.Msg == WM_MSG) // Catch all char messages
        {
            char c = (char)m.WParam;
            // Else just append char to barcodeBuilder to generate barcode
            barcodeBuilder.Append(c);
            return true;
        }

        return false;
    }

    #endregion

    private void RaiseBarcodeReadyEventAsync(string barcode)
    {
        Task.Factory.StartNew(() =>
        {
            try
            {
                // Generate barcode
                Console.WriteLine("F13 activated barcode [" + barcode + "]");

                if (BarcodeScannerReady != null)
                {
                    BarcodeScannerReady(this, new BarcodeScannerReadyEventArgs(barcode));
                }
            }
            catch (Exception ex)
            {
                // Do some error logging if needed
                Console.WriteLine(ex);
            }
        });
    }
}

public class BarcodeScannerReadyEventArgs : EventArgs
{
    public string Barcode { get; private set; }

    public BarcodeScannerReadyEventArgs(string barcode)
    {
        this.Barcode = barcode;
    }
}

I hope this helps others struggling with issues like mine.