Mystery nullReferenceException从Xamarin.Forms从4.1升级到4.2+后发生

问题描述 投票：0回答：1

将我的应用程序从Xamarin.Forms 4.1.0.709244升级到4.2.0.709249（或更新的任何软件包版本）后，以前可以正常运行的代码现在不再起作用并崩溃，但只能在一种具体情况。

我的应用程序通过OneSignal包接收推送通知，预期的行为是：

  1. 通知到达并打开
  2. 通知打开的处理程序将有效负载保存到App类参数并设置Application.Current.MainPage = new MainPage();
  3. MainPage初始化程序内部的逻辑解析有效负载以确定用户需要导航到的页面，并为该新页面创建一个Page对象。
  4. [Detail页面被设置为新页面

在4.1]下，此功能按预期工作。单击该通知，将在所有情况下将用户带到应用程序中的相应页面。升级到4.2 +后，如果该应用当前未运行，则此方案失败，但是如果该应用正在运行且处于前台或后台，则按预期方式工作。

[失败时，不是加载应用程序，而是将用户带到正确的页面，而是加载了应用程序，但随后它位于默认的主页上（好像未使用Detail中的信息设置通知有效负载）。同样，在这一点上，如果您点击汉堡包图标，导航菜单将短暂显示，并且应用程序立即崩溃，并出现空引用异常，如下所示：

System.NullReferenceException：对象引用未设置为对象的实例在Xamarin.Forms.Platform.Android.AppCompat.Platform.Xamarin.Forms.Platform.Android.IPlatformLayout.OnLayout（System.Boolean已更改，System.Int32 l，System.Int32 t，System.Int32 r，System.Int32 b） <596751900f1f46919eb25349c2e7053a>：0中的[0x0002b]在Xamarin.Forms.Platform.Android.PlatformRenderer.OnLayout（System.Boolean更改，System.Int32 l，System.Int32 t，System.Int32 r，System.Int32 b）[0x00025]在<596751900f1f46919eb25349c2e7053a>：0中在Android.Views.ViewGroup.n_OnLayout_ZIIII（System.IntPtr jnienv，System.IntPtr native__this，System.Boolean更改，System.Int32 l，System.Int32 t，System.Int32 r，System.Int32 b）[0x00009]在<21b22bf2aca24508938d2117f4c >：0在（包装器动态方法）Android.Runtime.DynamicMethodNameCounter.28（intptr，intptr，bool，int，int，int，int）

因为这种情况仅在应用程序通过通知操作关闭和启动时发生，所以我无法使用Visual Studio

调试器来准确确定发生异常的位置。我使用Microsoft AppCenter添加了许多事件，这些事件将跟踪各种方法的进度，然后报告崩溃在逻辑流程中的何处发生，但是所有预期的步骤都在进行中。正在调用将Detail页面设置为新页面的步骤，并且在点击汉堡图标时调用的isPresentedChanged处理程序也正在完成。

我已经查看了[[4.2

here]的发行说明，虽然有一些与页面布局有关的修复可能与崩溃有关，但我感到崩溃更像是更改的症状到“详细信息”页面无法正常进行，而不是我需要彻底解决的问题。我发现向Xamarin团队报告了一些听起来相近的错误，但已将它们解决为已解决，并且不是完全相同的问题。有关这些信息，请参见here，here，here，here和here。

NotificationServices.cs

（OneSignal的服务扩展；无论传递哪个userNotificationType，现在代码都失败：]public class NotificationServices { public static void HandleNotificationOpened(OSNotificationOpenedResult result) { string notificationActionId = "HandleMessage"; OSNotificationPayload payload = result.notification.payload; string message = payload.body; App.NotificationActionId = notificationActionId; App.NotificationData = payload.additionalData; Application.Current.MainPage = new MainPage(); } public static Page GetPageFromNotificationData() { Page ReturnPage; ApiServices _apiServices = new ApiServices(); //service to make REST calls to backend system user_notification_type userNotificationType = user_notification_type.None; int fromUserId = 0; int assocId = 0; int msgId = 0; if (App.NotificationData != null) { if (App.NotificationData.ContainsKey("assocId")) { Int32.TryParse(Convert.ToString(App.NotificationData["assocId"]), out assocId); } if (App.NotificationData.ContainsKey("fromUserId")) { Int32.TryParse(Convert.ToString(App.NotificationData["fromUserId"]), out fromUserId); } if (App.NotificationData.ContainsKey("msgId")) { Int32.TryParse(Convert.ToString(App.NotificationData["msgId"]), out msgId); } if (App.NotificationData.ContainsKey("userNotificationType")) { int unInt = 0; Int32.TryParse(Convert.ToString(App.NotificationData["userNotificationType"]), out unInt); userNotificationType = (user_notification_type)unInt; } } switch (userNotificationType) { case user_notification_type.ChatMessage: TeamBasic tm = new TeamBasic(); tm.OwnerID = fromUserId; tm.OwnerName = _apiServices.GetUserName(fromUserId).Result; ReturnPage = new ChatPage(tm); break; case user_notification_type.None: ReturnPage = default(Page); break; default: UserNotification unItem = new UserNotification(); var unList = _apiServices.GetUserNotifications(assocId, msgId+1, 1).Result; unItem = unList[0]; UserNotificationDetailViewModel undVm = new UserNotificationDetailViewModel(unItem); ReturnPage = new UserNotificationDetailPage(undVm); break; } return ReturnPage; } } 
Mainpage.xaml.cs
：public partial class MainPage : MasterDetailPage { private IHubServices _hubServices; Dictionary<int, NavigationPage> MenuPages = new Dictionary<int, NavigationPage>(); public MainPage() { InitializeComponent(); _hubServices = DependencyService.Get<IHubServices>(); //signalR _hubServices.Connect(); _hubServices.ClearPageCache += ClearPageCache; MasterBehavior = MasterBehavior.Popover; NavigationPage navPage = (NavigationPage)Detail; ConnectionState cs = _hubServices.GetConnectionState().Result; if (cs == ConnectionState.Connected) { var pg = NotificationServices.GetPageFromNotificationData(); //call to get page as specified per notification Detail = new NavigationPage(pg); //this is where the Detail page should be getting updated but is acting like it isn't when the app is started by the act of opening the notification MenuPages.Add((int)MenuItemType.LogOut, (NavigationPage)Detail); } else { App.CheckForNotificationRedirect = true; MenuPages.Add((int)MenuItemType.About, (NavigationPage)Detail); } IsPresentedChanged += (sender, args) => { try { //anything } catch (Exception ex) { //anything } //it is after this has completed executing that the app is crashing. No exception ever occurs in the try/catch }; } } 
由[[App Center
提供的完整转储：

Xamarin异常堆栈：System.NullReferenceException：对象引用未设置为对象的实例在Xamarin.Forms.Platform.Android.AppCompat.Platform.Xamarin.Forms.Platform.Android.IPlatformLayout.OnLayout（System.Boolean已更改，System.Int32 l，System.Int32 t，System.Int32r，System.Int32 b）[0x0002b]在<596751900f1f46919eb25349c2e7053a>：0中在Xamarin.Forms.Platform.Android.PlatformRenderer.OnLayout（System.Boolean已更改，System.Int32 l，System.Int32 t，System.Int32r，System.Int32 b）<596751900f1f46919eb25349c2e7053a>：0中的[0x00025]在Android.Views.ViewGroup.n_OnLayout_ZIIII（System.IntPtr jnienv，System.IntPtr native__this，System.Boolean更改，System.Int32 l，System.Int32 t，System.Int32 r，System.Int32 b）<21b22bf2aca24508938d2117f4c11761>中的[0x00009]：0在（包装器动态方法）Android.Runtime.DynamicMethodNameCounter.28（intptr，intptr，bool，int，int，int，int）

Thread 2: 0 dalvik.system.VMStack.getThreadStackTrace(VMStack.java:-2) 1 java.lang.Thread.getStackTrace(Thread.java:1538) 2 java.lang.Thread.getAllStackTraces(Thread.java:1588) 3 com.microsoft.appcenter.crashes.Crashes.saveUncaughtException(Crashes.java:1093) 4 com.microsoft.appcenter.crashes.WrapperSdkExceptionManager.saveWrapperException(WrapperSdkExceptionManager.java:58) 5 crc643f46942d9dd1fff9.PlatformRenderer.n_onLayout(PlatformRenderer.java:-2) 6 crc643f46942d9dd1fff9.PlatformRenderer.onLayout(PlatformRenderer.java:55) 7 android.view.View.layout(View.java:20740) 8 android.view.ViewGroup.layout(ViewGroup.java:6268) 9 android.widget.RelativeLayout.onLayout(RelativeLayout.java:1084) 10 android.view.View.layout(View.java:20740) 11 android.view.ViewGroup.layout(ViewGroup.java:6268) 12 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 13 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 14 android.view.View.layout(View.java:20740) 15 android.view.ViewGroup.layout(ViewGroup.java:6268) 16 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 17 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 18 android.view.View.layout(View.java:20740) 19 android.view.ViewGroup.layout(ViewGroup.java:6268) 20 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 21 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 22 android.view.View.layout(View.java:20740) 23 android.view.ViewGroup.layout(ViewGroup.java:6268) 24 android.widget.LinearLayout.setChildFrame(LinearLayout.java:1812) 25 android.widget.LinearLayout.layoutVertical(LinearLayout.java:1656) 26 android.widget.LinearLayout.onLayout(LinearLayout.java:1565) 27 android.view.View.layout(View.java:20740) 28 android.view.ViewGroup.layout(ViewGroup.java:6268) 29 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 30 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 31 com.android.internal.policy.DecorView.onLayout(DecorView.java:794) 32 android.view.View.layout(View.java:20740) 33 android.view.ViewGroup.layout(ViewGroup.java:6268) 34 android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2970) 35 android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2465) 36 android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1571) 37 android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7616) 38 android.view.Choreographer$CallbackRecord.run(Choreographer.java:1034) 39 android.view.Choreographer.doCallbacks(Choreographer.java:845) 40 android.view.Choreographer.doFrame(Choreographer.java:780) 41 android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1020) 42 android.os.Handler.handleCallback(Handler.java:873) 43 android.os.Handler.dispatchMessage(Handler.java:99) 44 android.os.Looper.loop(Looper.java:205) 45 android.app.ActivityThread.main(ActivityThread.java:6991) 46 java.lang.reflect.Method.invoke(Method.java:-2) 47 com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 48 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:884) Thread 41398: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Daemons$ReferenceQueueDaemon.runInternal(Daemons.java:178) 2 java.lang.Daemons$Daemon.run(Daemons.java:103) 3 java.lang.Thread.run(Thread.java:764) Thread 41399: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Object.wait(Object.java:422) 2 java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:188) 3 java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:209) 4 java.lang.Daemons$FinalizerDaemon.runInternal(Daemons.java:232) 5 java.lang.Daemons$Daemon.run(Daemons.java:103) 6 java.lang.Thread.run(Thread.java:764) Thread 41400: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Daemons$FinalizerWatchdogDaemon.sleepUntilNeeded(Daemons.java:297) 2 java.lang.Daemons$FinalizerWatchdogDaemon.runInternal(Daemons.java:277) 3 java.lang.Daemons$Daemon.run(Daemons.java:103) 4 java.lang.Thread.run(Thread.java:764) Thread 41415: 0 java.lang.Object.wait(Object.java:-2) 1 com.android.okhttp.ConnectionPool$1.run(ConnectionPool.java:103) 2 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 3 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 4 java.lang.Thread.run(Thread.java:764) Thread 41416: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41417: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41419: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41420: 0 java.lang.Object.wait(Object.java:-2) 1 com.android.okhttp.okio.AsyncTimeout.awaitTimeout(AsyncTimeout.java:323) 2 com.android.okhttp.okio.AsyncTimeout.access$000(AsyncTimeout.java:40) 3 com.android.okhttp.okio.AsyncTimeout$Watchdog.run(AsyncTimeout.java:286) Thread 41421: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41424: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41425: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41431: 0 android.os.MessageQueue.nativePollOnce(MessageQueue.java:-2) 1 android.os.MessageQueue.next(MessageQueue.java:326) 2 android.os.Looper.loop(Looper.java:170) 3 android.os.HandlerThread.run(HandlerThread.java:65) Thread 41469: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Thread.parkFor$(Thread.java:2137) 2 sun.misc.Unsafe.park(Unsafe.java:358) 3 java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:230) 4 java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2101) 5 java.util.concurrent.LinkedBlockingQueue.poll(LinkedBlockingQueue.java:467) 6 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1091) 7 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1152) 8 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 9 com.google.android.gms.common.util.concurrent.zza.run 10 java.lang.Thread.run(Thread.java:764) Thread 41470: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Thread.parkFor$(Thread.java:2137) 2 sun.misc.Unsafe.park(Unsafe.java:358) 3 java.util.concurrent.locks.LockSupport.park(LockSupport.java:190) 4 java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2059) 5 java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1120) 6 java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:849) 7 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1092) 8 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1152) 9 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 10 com.google.android.gms.common.util.concurrent.zza.run 11 java.lang.Thread.run(Thread.java:764) Thread 41478: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Thread.parkFor$(Thread.java:2137) 2 sun.misc.Unsafe.park(Unsafe.java:358) 3 java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:230) 4 java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2101) 5 java.util.concurrent.LinkedBlockingQueue.poll(LinkedBlockingQueue.java:467) 6 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1091) 7 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1152) 8 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 9 java.lang.Thread.run(Thread.java:764) Thread 41483: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Thread.parkFor$(Thread.java:2137) 2 sun.misc.Unsafe.park(Unsafe.java:358) 3 java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:230) 4 java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2101) 5 java.util.concurrent.LinkedBlockingQueue.poll(LinkedBlockingQueue.java:467) 6 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1091) 7 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1152) 8 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 9 java.lang.Thread.run(Thread.java:764) Thread 41495: 0 java.lang.Object.wait(Object.java:-2) 1 java.lang.Thread.parkFor$(Thread.java:2137) 2 sun.misc.Unsafe.park(Unsafe.java:358) 3 java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:230) 4 java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2101) 5 java.util.concurrent.LinkedBlockingQueue.poll(LinkedBlockingQueue.java:467) 6 java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1091) 7 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1152) 8 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 9 java.lang.Thread.run(Thread.java:764)

将我的应用程序从Xamarin.Forms 4.1.0.709244升级到4.2.0.709249（或任何更新的软件包版本后，以前可以正常运行的代码现在不再有效并崩溃，但只能在一个...下运行）]] >> 

经过大量的努力来查找触发代码之后，确定需要将服务类中的以下行包装在调用中，以确保在主线程上被调用。在Xamarin.Forms 4.2之前不需要这样做。
await Device.InvokeOnMainThreadAsync(() => {
    Application.Current.MainPage = new MainPage();
});
android xamarin.forms onesignal
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