异常：System.NullReferenceException：'Object reference not set to an instance of an object.'

Question

我正在尝试创建一个简单的 Qr 码扫描器应用程序，它有一个像这样的单一视图：

但是当我尝试按“点击”按钮启动扫描仪时，我得到了这个异常：

System.NullReferenceException: 'Object reference not set to an instance of an object.'

这是 main.xaml 页面：

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="scanner.MainPage">

        <StackLayout Spacing="10">
            <Button Text="Click"  
                x:Name="btnScan"  
                Clicked="btnScan_Clicked"/>
            <Entry x:Name="txtBarcode"  
               Placeholder="Text Do scan"/>
        </StackLayout>

</ContentPage>

main.xaml.cs:

 public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private async void btnScan_Clicked(object sender, EventArgs e)
        {
            try
            {
                var scanner = DependencyService.Get<Interface1>();
                var result = await scanner.ScanAsync();
                if (result != null)
                {
                    txtBarcode.Text = result;
                }
            }
            catch (Exception ex)
            { throw;
            }

服务：

  internal class QrScanningService : Interface1
    {
        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;
        }
    }

Answer 1

我做了一个简单的，遇到了同样的问题。于是在网上搜索了一下，发现使用ZXing.Net.Mobile包时需要初始化插件

所以可以在MainActivity的OnCreate方法中加入初始化代码。如：

 protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        MobileBarcodeScanner.Initialize(Application);// this line initialize the plugin
        Xamarin.Essentials.Platform.Init(this, savedInstanceState);
        global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
        LoadApplication(new App());
    }

你可以查看文档:https://github.com/Redth/ZXing.Net.Mobile#android

然后在 ios 上检查这个 link:https://github.com/Redth/ZXing.Net.Mobile#ios

异常：System.NullReferenceException：'Object reference not set to an instance of an object.'

Exception : System.NullReferenceException: 'Object reference not set to an instance of an object.'

xamarin

qr-code