VS2022下Xamarin Forms Android NFC标签读取异常求助

1. 补全AndroidManifest.xml配置 #

当前权限配置缺少NFC事件的Intent过滤规则，系统无法将NFC标签发现事件传递给应用。需要在<activity>标签内添加以下内容：

<intent-filter>
    <action android:name="android.nfc.action.NDEF_DISCOVERED" />
    <action android:name="android.nfc.action.TECH_DISCOVERED" />
    <action android:name="android.nfc.action.TAG_DISCOVERED" />
    <category android:name="android.intent.category.DEFAULT" />
</intent-filter>
<meta-data android:name="android.nfc.action.TECH_DISCOVERED" android:resource="@xml/nfc_tech_filter" />

同时在Resources/xml目录下创建nfc_tech_filter.xml文件（若xml目录不存在需手动创建），内容为：

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <tech-list>
        <tech>android.nfc.tech.IsoDep</tech>
        <tech>android.nfc.tech.NfcA</tech>
        <tech>android.nfc.tech.NfcB</tech>
        <tech>android.nfc.tech.NfcF</tech>
        <tech>android.nfc.tech.NfcV</tech>
        <tech>android.nfc.tech.Ndef</tech>
        <tech>android.nfc.tech.NdefFormatable</tech>
        <tech>android.nfc.tech.MifareClassic</tech>
        <tech>android.nfc.tech.MifareUltralight</tech>
    </tech-list>
</resources>

2. 完善MainActivity代码 #

Android系统通过Intent传递NFC事件，需重写OnNewIntent方法将事件转发给Plugin.NFC：

using Android.App;
using Android.Content.PM;
using Android.Runtime;
using Android.OS;
using Plugin.NFC;
using Android.Content;

namespace Scan_NFC.Droid
{
    [Activity(Label = "Scan_NFC", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize )]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            CrossNFC.Init(this);

            Xamarin.Essentials.Platform.Init(this, savedInstanceState);
            global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
            LoadApplication(new App());
        }

        protected override void OnNewIntent(Intent intent)
        {
            base.OnNewIntent(intent);
            CrossNFC.Current.OnNewIntent(intent);
        }

        public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
        {
            Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);
            base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }
}

3. 修复MainPage代码中的冗余与日志排查 #

• 移除构造函数中手动查找meuEntry的代码：meuEntry = (Entry)FindByName("meuEntry");，Xamarin会自动为x:Name生成对应的字段，手动赋值可能导致引用异常。
• 添加日志输出，确认NFC事件是否触发：

using Plugin.NFC;
using System;
using Xamarin.Forms;

namespace Scan_NFC
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
            CrossNFC.Current.OnMessageReceived += NfcMessageReceived;
            CrossNFC.Current.OnTagListeningStatusChanged += (isListening) => 
                System.Diagnostics.Debug.WriteLine(isListening ? "NFC监听已启动" : "NFC监听已停止");
        }

        private void NfcMessageReceived(ITagInfo tagInfo)
        {
            System.Diagnostics.Debug.WriteLine("已触发NFC消息接收事件");
            if (tagInfo == null)
            {
                System.Diagnostics.Debug.WriteLine("TagInfo为空");
                return;
            }

            var idBytes = tagInfo.Identifier;
            var message = BitConverter.ToString(idBytes);
            System.Diagnostics.Debug.WriteLine($"读取到NFC标签ID：{message}");

            Device.BeginInvokeOnMainThread(() =>
            {
                meuEntry.Text = $"Tag NFC Lida - ID: {message}";
                DisplayAlert("Tag NFC Lida", $"ID: {message}", "OK");
            });
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();
            System.Diagnostics.Debug.WriteLine("MainPage OnAppearing");
            CrossNFC.Current.StartListening(new System.Collections.Generic.List<string>(), false);
        }

        protected override void OnDisappearing()
        {
            base.OnDisappearing();
            System.Diagnostics.Debug.WriteLine("MainPage OnDisappearing");
            CrossNFC.Current.StopListening();
        }
    }
}

4. 验证Plugin.NFC版本 #

确保项目中引用的Plugin.NFC包是最新稳定版，兼容当前Xamarin Forms版本，避免版本不匹配导致的事件不触发问题。

完成以上修改后，重新部署应用，测试NFC标签读取：

• 查看输出窗口的日志，确认是否打印"已触发NFC消息接收事件"和标签ID
• 若日志正常输出但Entry仍不更新，检查meuEntry的绑定是否正确，或尝试用Label替代Entry测试UI更新是否正常

内容的提问来源于stack exchange，提问作者Eduardo Elias