订阅通知接口导致Xamarin Forms Android应用崩溃问题求助

你遇到的空引用异常核心是 DependencyService.Get<INotificationManager>() 返回了null，触发原因如下：

• 未在Android平台项目中实现INotificationManager接口
• 实现类未注册到Xamarin.Forms依赖服务容器
• 小概率是接口定义位置错误（放在了平台项目而非共享项目）

1. 补充缺失的公共类与平台实现 #

首先在共享项目中补充NotificationEventArgs类定义（你代码中用到了该类但未声明）：

public class NotificationEventArgs : EventArgs
{
    public string Title { get; set; }
    public string Message { get; set; }
}

然后在Android平台项目中新建AndroidNotificationManager.cs类，实现接口并注册依赖服务：

using Android.App;
using Android.Content;
using Android.OS;
using AndroidX.Core.App;
using Xamarin.Forms;
// 必须加这行注册依赖服务，写在命名空间外
[assembly: Dependency(typeof(你的Android项目命名空间.AndroidNotificationManager))]

namespace 你的Android项目命名空间
{
    public class AndroidNotificationManager : INotificationManager
    {
        public const string TitleKey = "title";
        public const string MessageKey = "message";
        const string channelId = "default";
        const string channelName = "Default";
        const string channelDescription = "Default channel for notifications";
        int messageId = 0;

        public event EventHandler NotificationReceived;

        public void Initialize()
        {
            CreateNotificationChannel();
        }

        public void SendNotification(string title, string message, DateTime? notifyTime = null)
        {
            var intent = new Intent(Android.App.Application.Context, typeof(MainActivity));
            intent.PutExtra(TitleKey, title);
            intent.PutExtra(MessageKey, message);
            intent.AddFlags(ActivityFlags.SingleTop);

            var pendingIntent = PendingIntent.GetActivity(Android.App.Application.Context, 0, intent, PendingIntentFlags.UpdateCurrent | PendingIntentFlags.Immutable);
            
            var notificationBuilder = new NotificationCompat.Builder(Android.App.Application.Context, channelId)
                .SetContentTitle(title)
                .SetContentText(message)
                .SetSmallIcon(Resource.Mipmap.icon)
                .SetContentIntent(pendingIntent)
                .SetAutoCancel(true);

            var notificationManager = NotificationManager.FromContext(Android.App.Application.Context);
            notificationManager.Notify(messageId++, notificationBuilder.Build());
        }

        public void ReceiveNotification(string title, string message)
        {
            NotificationReceived?.Invoke(this, new NotificationEventArgs { Title = title, Message = message });
        }

        void CreateNotificationChannel()
        {
            if (Build.VERSION.SdkInt < BuildVersionCodes.O)
            {
                return;
            }
            var channel = new NotificationChannel(channelId, channelName, NotificationImportance.Default)
            {
                Description = channelDescription
            };
            var notificationManager = NotificationManager.FromContext(Android.App.Application.Context);
            notificationManager.CreateNotificationChannel(channel);
        }
    }
}

注意替换代码中的你的Android项目命名空间为你实际Android项目的命名空间。

2. 补充空值校验避免崩溃 #

修改你的订阅逻辑代码，增加空校验，同时主动调用Initialize方法初始化通知通道：

INotificationManager notificationManager;
int notificationNumber = 0;

notificationManager = DependencyService.Get<INotificationManager>();
// 增加空校验
if (notificationManager != null)
{
    // 初始化通知通道
    notificationManager.Initialize();
    notificationManager.NotificationReceived += (sender, eventArgs) =>
    {
        var evtData = (NotificationEventArgs)eventArgs;
        ShowNotification(evtData.Title, evtData.Message);
    };
}

void ShowNotification(string title, string message)
{
    Device.BeginInvokeOnMainThread(() =>
    {
        var msg = new Label()
        {
            Text = $"Notification Received:\nTitle: {title}\nMessage: {message}"
        };
        stacklayout.Children.Add(msg);
    });
}

3. 适配高版本Android权限 #

如果你的App targets Android 13（API 33）及以上版本，需要在AndroidManifest.xml中添加通知权限声明：

<uses-permission android:name="android.permission.POST_NOTIFICATIONS" />

且在发送通知前主动申请该运行时权限。

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