Backbutton on ZXing Scanner doesn´t works (Xamarin Forms)

Question

im trying to implement a Barcode Scanner on my project in Xamarin Forms. When i press the back button on the scanning page my app crash. I am getting the following error: System.NullReferenceException: 'Object reference not set to an instance of an object.'

This is my code:

ScanService.cs

public interface ScanningService
{
    Task<string> ScanAsync();

}

My code behind:

var scanner = DependencyService.Get<ScanningService>();
        var result = await scanner.ScanAsync();
        if (result != null){.....}

QRscanningService (On Android):

   class QrScanningService : ScanningService
    {
       

        public async Task<string> ScanAsync()
        {

            var optionsCustom = new MobileBarcodeScanningOptions();

            var scanner = new MobileBarcodeScanner()
            {
                TopText = "Acerca la cámara",
                BottomText = "Toca la pantalla para enfocar",

            };
            var scanResult = await scanner.Scan(optionsCustom);

            return scanResult.Text;
        }

    }

Any contribution will be apreciated.

Which line causes the crash? What does the stack trace show? — Jason, Feb 04 '22 at 16:14
before i click the backbutton on my phone, on line if (result != null) { — Jair Olivares, Feb 08 '22 at 14:30
Add some exception handling. Learn to use the debugger. As I already suggested, look at the stack trace. A NullRef is a very common C# problem and there are a huge number of existing posts, tutorials, etc on how to debug them — Jason, Feb 08 '22 at 14:37

score 0 · Answer 1 · answered Feb 17 '22 at 09:00

I believe the error occurs inside QRscanningService (On Android).

It is because you're trying to use "Text" from scanResult and scanResult is null

To avoid this, change the last line to something like this:

return scanResult == null ? null: scanResult.Text;

Jair Olivares · Accepted Answer · 2022-03-08T22:44:45.350

I solved it with the following code on my code behind:

var overlay = new ZXingDefaultOverlay
{
    ShowFlashButton = true,
    TopText = "Toca la pantalla para enfocar",
    BottomText = string.Empty
};

overlay.BindingContext = overlay;

scanPage = new ZXingScannerPage(null, overlay);
            
overlay.FlashButtonClicked += (s, ed) =>
{
    scanPage.ToggleTorch();
};

scanPage.OnScanResult += (result) =>
{
    scanPage.IsScanning = false;

    Device.BeginInvokeOnMainThread(() =>
    {
        _ = Navigation.PopModalAsync();
        try
        {
            TxtidR.Text = result.Text;
        }
        catch (Exception)
        {
            throw;
        }
    });
};

await Navigation.PushModalAsync(scanPage);
var scanner = DependencyService.Get<ScanningService>();
}

I hope it works for you!

Backbutton on ZXing Scanner doesn´t works (Xamarin Forms)

2 Answers2