Xamarin forms: System.NullReferenceException when using ZXing.Net.Mobile for scanning?

Question

I am using the ZXing.Net.Mobile NuGet package for scanning barcode numbers. I have done everything as per this blog.

My Code

//Main Project Interface
public interface IQrScanningService  
{  
   Task<string> ScanAsync();  
}

//Android part implementation
[assembly: Dependency(typeof(XFBarcode.Droid.Services.QrScanningService))]  
namespace projectname.Droid.Services  
{  
    public class QrScanningService : IQrScanningService  
    {  
        public async Task<string> ScanAsync()  
        {  
            var optionsDefault = new MobileBarcodeScanningOptions();  
            var optionsCustom = new MobileBarcodeScanningOptions();  

            var scanner = new MobileBarcodeScanner()  
            {  
                TopText = "Scan the QR Code",  
                BottomText = "Please Wait",  
            };  

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

But when I execute I am getting System.NullReferenceException: 'Object reference not set to an instance of an object.'. What else I am missing here? I saw a similar thread here but don't how to download use a package from GitHub.

Answer 1

Hello,

Welcome to Microsoft Q&A!

The error is happening because you're using a null object scanner .

Before using the plugin we need to initialize the plugin .

   MobileBarcodeScanner.Initialize(MainActivity.Instance.Application);  
     
     
   //In your MainActivity   
     
   public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity  
   {  
       public static MainActivity Instance;  
       protected override void OnCreate(Bundle savedInstanceState)  
       {  
           TabLayoutResource = Resource.Layout.Tabbar;  
           ToolbarResource = Resource.Layout.Toolbar;  
     
           base.OnCreate(savedInstanceState);  
           Instance = this;  
           Xamarin.Essentials.Platform.Init(this, savedInstanceState);  
             
           global::Xamarin.Forms.Forms.Init(this, savedInstanceState);  
           LoadApplication(new App());  
       }  
     
   }  

Or initialize directly in MainActivity OnCreate() method .

   protected override void OnCreate(Bundle savedInstanceState)  
   {  
       TabLayoutResource = Resource.Layout.Tabbar;  
       ToolbarResource = Resource.Layout.Toolbar;  
     
       base.OnCreate(savedInstanceState);  
       Xamarin.Essentials.Platform.Init(this, savedInstanceState);  
     
     
       MobileBarcodeScanner.Initialize(Application); //initialize  here  
     
     
       global::Xamarin.Forms.Forms.Init(this, savedInstanceState);  
       LoadApplication(new App());  
   }  

Refer to https://github.com/Redth/ZXing.Net.Mobile#usage .

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