Winforms keypress and barcode scanner

Question

I managed using bar-code scanner successfully in my WPF project using a keyboard hook as follows (I skip some details, but basically, I can rely on the fact that I know which keyboard is my scanner).

/// <summary>
/// Add this KeyboardHook to a window
/// </summary>
/// <param name="window">The window to add to</param>
public void AddHook(Window window) {
  if (form == null)
    throw new ArgumentNullException("window");
  if (mHwndSource != null)
    throw new InvalidOperationException("Hook already present");

  WindowInteropHelper w = new WindowInteropHelper(window);
  IntPtr hwnd = w.Handle;
  mHwndSource = HwndSource.FromHwnd(hwnd);
  if (mHwndSource == null)
    throw new ApplicationException("Failed to receive window source");

  mHwndSource.AddHook(WndProc);
  
  RAWINPUTDEVICE[] rid = new RAWINPUTDEVICE[1];

  rid[0].usUsagePage = 0x01;
  rid[0].usUsage = 0x06;
  rid[0].dwFlags = RIDEV_INPUTSINK;
  rid[0].hwndTarget = hwnd;

  if (!RegisterRawInputDevices(rid, (uint)rid.Length, (uint)Marshal.SizeOf(rid[0])))
    throw new ApplicationException("Failed to register raw input device(s).");
}

The approach then processes WM_INPUT messages to retrieve information about any keyboard events that occur and handles the event accordingly if it is coming from the Bar-code scanner that is already known.

Now the thing is that in Winforms I should not use hooks but override WndProc as stated here, but I am somehow struggling to understand how I can use WndProc as I need to know:

a) what event I really need to handle in the WndProc method

b) how I can identify the device that fired the event

Any help would be very appreciated! Cheers!

Answer 1

I ended using the following approach:

public class BarcodeScannedEventArgs : EventArgs {

    public BarcodeScannedEventArgs(string text) {
      mText = text;
    }
    public string ScannedText { get { return mText; } }

    private readonly string mText;
  }

  public class BarCodeListener : IDisposable {
    DateTime _lastKeystroke = new DateTime(0);
    string _barcode = string.Empty;
    Form _form;
    bool isKeyPreview;

    public bool ProcessCmdKey(ref Message msg, Keys keyData) {
      bool res = processKey(keyData);
      return keyData == Keys.Enter ? res : false;
    }

    protected bool processKey(Keys key) {
      // check timing (>7 keystrokes within 50 ms ending with "return" char)
      TimeSpan elapsed = (DateTime.Now - _lastKeystroke);
      if (elapsed.TotalMilliseconds > 50) {
        _barcode = string.Empty;
      }

      // record keystroke & timestamp -- do NOT add return at the end of the barcode line
      if (key != Keys.Enter) {
        _barcode += (char)key;
      }
      _lastKeystroke = DateTime.Now;

      // process barcode only if the return char is entered and the entered barcode is at least 7 digits long.
      // This is a "magical" rule working well for EAN-13 and EAN-8, which both have at least 8 digits...
      if (key == Keys.Enter && _barcode.Length > 7) {
        if (BarCodeScanned != null) {
          BarCodeScanned(_form, new BarcodeScannedEventArgs(_barcode));
        }
        _barcode = string.Empty;
        return true;
      }
      return false;
    }

    public event EventHandler<BarcodeScannedEventArgs> BarCodeScanned;

    public BarCodeListener(Form f) {
      _form = f;
      isKeyPreview = f.KeyPreview;
      // --- set preview and register event...
      f.KeyPreview = true;
    }

    public void Dispose() {
      if (_form != null) {
        _form.KeyPreview = isKeyPreview;
        //_form.KeyPress -= KeyPress_scanner_preview;
      }
    }
  }
}

Then, add the following lines of code to your form which is listening to your scanner:

private BarCodeListener ScannerListener;
protected override bool ProcessCmdKey(ref Message msg, Keys keyData) {
  bool res = false;
  if (ScannerListener != null) {
    res = ScannerListener.ProcessCmdKey(ref msg, keyData);
  }
  res = keyData == Keys.Enter ? res : base.ProcessCmdKey(ref msg, keyData);
  return res;
}