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How to distinguish between multiple input devices in C#

由 半城伤御伤魂 提交于 2019-11-26 20:22:07

You could use the Raw Input API to distinguish between the keyboard and the scanner like I did recently. It doesn't matter how many keyboard or keyboard-like devices you have hooked up; you will see a WM_INPUT before the keystroke is mapped to a device-independent virtual key that you typically see in a KeyDown event.

Far easier is to do what others have recommended and configure the scanner to send sentinel characters before and after the barcode. (You usually do this by scanning special barcodes in the back of the scanner's user manual.) Then, your main form's KeyPreview event can watch those roll end and swallow the key events for any child control if it's in the middle of a barcode read. Or, if you wanted to be fancier, you could use a low-level keyboard hook with SetWindowsHookEx() to watch for those sentinels and swallow them there (advantage of this is you could still get the event even if your app didn't have focus).

I couldn't change the sentinel values on our barcode scanners among other things so I had to go the complicated route. Was definitely painful. Keep it simple if you can!

--

Your update, seven years later: If your use case is reading from a USB barcode scanner, Windows 10 has a nice, friendly API for this built-in in Windows.Devices.PointOfService.BarcodeScanner. It's a UWP/WinRT API, but you can use it from a regular desktop app as well; that's what I'm doing now. Here's some example code for it, straight from my app, to give you the gist:

{
    using System;
    using System.Linq;
    using System.Threading.Tasks;
    using System.Windows;
    using Windows.Devices.Enumeration;
    using Windows.Devices.PointOfService;
    using Windows.Storage.Streams;
    using PosBarcodeScanner = Windows.Devices.PointOfService.BarcodeScanner;

    public class BarcodeScanner : IBarcodeScanner, IDisposable
    {
        private ClaimedBarcodeScanner scanner;

        public event EventHandler<BarcodeScannedEventArgs> BarcodeScanned;

        ~BarcodeScanner()
        {
            this.Dispose(false);
        }

        public bool Exists
        {
            get
            {
                return this.scanner != null;
            }
        }

        public void Dispose()
        {
            this.Dispose(true);
            GC.SuppressFinalize(this);
        }

        public async Task StartAsync()
        {
            if (this.scanner == null)
            {
                var collection = await DeviceInformation.FindAllAsync(PosBarcodeScanner.GetDeviceSelector());
                if (collection != null && collection.Count > 0)
                {
                    var identity = collection.First().Id;
                    var device = await PosBarcodeScanner.FromIdAsync(identity);
                    if (device != null)
                    {
                        this.scanner = await device.ClaimScannerAsync();
                        if (this.scanner != null)
                        {
                            this.scanner.IsDecodeDataEnabled = true;
                            this.scanner.ReleaseDeviceRequested += WhenScannerReleaseDeviceRequested;
                            this.scanner.DataReceived += WhenScannerDataReceived;

                            await this.scanner.EnableAsync();
                        }
                    }
                }
            }
        }

        private void WhenScannerDataReceived(object sender, BarcodeScannerDataReceivedEventArgs args)
        {
            var data = args.Report.ScanDataLabel;

            using (var reader = DataReader.FromBuffer(data))
            {
                var text = reader.ReadString(data.Length);
                var bsea = new BarcodeScannedEventArgs(text);
                this.BarcodeScanned?.Invoke(this, bsea);
            }
        }

        private void WhenScannerReleaseDeviceRequested(object sender, ClaimedBarcodeScanner args)
        {
            args.RetainDevice();
        }

        private void Dispose(bool disposing)
        {
            if (disposing)
            {
                this.scanner = null;
            }
        }
    }
}

Granted, you'll need a barcode scanner that supports the USB HID POS and isn't just a keyboard wedge. If your scanner is just a keyboard wedge, I recommend picking up something like a used Honeywell 4600G off eBay for like $25. Trust me, your sanity will be worth it.

What I did in a similar situation is distinguish between a scan and a user typing by looking at the speed of the input.

Lots of characters very close together then a pause is a scan. Anything else is keyboard input.

I don't know exactly your requirements, so maybe that won't do for you, but it's the best I've got :)

It depends on the way you are interacting with the device. Anyway it wont be a C# solution, it will be some other library. Are you reading data from a stream? If you are just taking keystrokes, there may be nothing you can do about it.

DieterF

I know this is an old thread, found it by searching barcode scanning in WIN10. Just a few notes in case someone needs it.

These scanners from Honeywell have several USB interfaces. One is a keyboard + Hid Point of sales (composite device). Also there are CDC-ACM (ComPort emulation) and Hid Point of sales (alone) + more.

By default the scanners expose a serial number, so the host can distinguish between many devices (I had once +20 connected). There is a command to disable the serial number though!

The newer models behave the same in this regard. If you want to see it live, try my terminal program yat3 (free on my site). It can open all the interfaces mentioned above and is tailored for such devices.

A word to use keyboard interfaces:
Only use them as a last resort. They are slow, less reliable when it comes to exotic characters. The only good use is if you want to enter data into existing applications. If you code anyway, then reading from ComPort/HidPos-Device is easier.

look at this: http://nate.dynalias.net/dev/keyboardredirector.rails (NOT AVAILABLE ANYMORE) works great!

Specify the keyboard and the keys you want to block, and it works like a charm!

Also take a look at this: http://www.oblita.com/interception.html You can create a C# wrapper for it - it also works like a charm..

I think you might be able to distinguish multiple keyboards through DirectX API, or if that doesn't work, through raw input API.

I have successfully accomplished what you folks are looking for here. I have an application that receives all barcode character data from a Honeywell/Metrologic barcode scanner. No other application on the system receives the data from the scanner, and the keyboard continues to function normally.

My application uses a combination of raw input and the dreaded low-level keyboard hook system. Contrary to what is written here, I found that the wm_input message is received before the keyboard hook function is called. My code to process the wm_input message basically sets a boolean variable to specify whether or not the received character is from the scanner. The keyboard hook function, called immediately after the wm_input is processed, swallows the scanner’s pseudo-keyboard data, preventing the data from being received by other applications.

The keyboard hook function has to be placed in an dll since you want to intercept all system keyboard messages. Also, a memory mapped file has to be used for the wm_input processing code to communicate with the dll.

标签
c#
barcode-scanner
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