.Net Maui ArgumentException: 'An item with the same key has already been added. Key: Microsoft.Maui.Controls.BindableProperty' - What does that mean?

Question

I'm really at a loss here.

I'm working on a .Net Maui Application (.Net 8.0) for Android and iOS for work. When I open a specific page I sometimes (like, maybe 1 out of 6-10 times) get an Error with this message: "System.ArgumentException: 'An item with the same key has already been added. Key: Microsoft.Maui.Controls.BindableProperty'". No further information... The call stack in VS points me to external code and googling keyword from the call stack didn't help me further.

Here's the call stack:

0xFFFFFFFFFFFFFFFF in Android.Runtime.RuntimeNativeMethods.monodroid_debugger_unhandled_exception	C#
0x1A in Android.Runtime.JNINativeWrapper._unhandled_exception at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:12,5	C#
0x1D in Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:23,26	C#
0xB in System.ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException<Microsoft.Maui.Controls.BindableProperty>	C#
0x137 in System.Collections.Generic.Dictionary<Microsoft.Maui.Controls.BindableProperty,Microsoft.Maui.Controls.BindableObject.BindablePropertyContext>.TryInsert	C#
0x4 in System.Collections.Generic.Dictionary<Microsoft.Maui.Controls.BindableProperty,Microsoft.Maui.Controls.BindableObject.BindablePropertyContext>.Add	C#
0x49 in Microsoft.Maui.Controls.BindableObject.CreateAndAddContext at /_/src/Controls/src/Core/BindableObject.cs:698,4	C#
0xD in Microsoft.Maui.Controls.BindableObject.GetOrCreateContext at /_/src/Controls/src/Core/BindableObject.cs:706,76	C#
0x102 in Microsoft.Maui.Controls.BindableObject.SetValueCore at /_/src/Controls/src/Core/BindableObject.cs:553,4	C#
0x5B in Microsoft.Maui.Controls.BindableObject.SetValue at /_/src/Controls/src/Core/BindableObject.cs:503,4	C#
0x55 in Microsoft.Maui.Controls.BindableObjectExtensions.RefreshPropertyValue at /_/src/Controls/src/Core/BindableObjectExtensions.cs:26,5	C#
0x26 in Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.Controls.IPropertyPropagationController.PropagatePropertyChanged at /_/src/Controls/src/Core/VisualElement/VisualElement.cs:1662,5	C#
0x29 in Microsoft.Maui.Controls.Element.OnParentSet at /_/src/Controls/src/Core/Element/Element.cs:627,4	C#
0x1 in Microsoft.Maui.Controls.NavigableElement.OnParentSet at /_/src/Controls/src/Core/Shell/NavigableElement.cs:97,4	C#
0x161 in Microsoft.Maui.Controls.Element.SetParent at /_/src/Controls/src/Core/Element/Element.cs:430,4	C#
0x2 in Microsoft.Maui.Controls.Element.OnChildAdded at /_/src/Controls/src/Core/Element/Element.cs:588,4	C#
0x2 in Microsoft.Maui.Controls.VisualElement.OnChildAdded at /_/src/Controls/src/Core/VisualElement/VisualElement.cs:1253,4	C#
0x1E in Microsoft.Maui.Controls.Compatibility.Layout.OnInternalAdded at /_/src/Controls/src/Core/Layout.cs:601,4	C#
0x88 in Microsoft.Maui.Controls.Compatibility.Layout.InternalChildrenOnCollectionChanged at /_/src/Controls/src/Core/Layout.cs:591,6	C#
0x1B in System.Collections.ObjectModel.ObservableCollection<Microsoft.Maui.Controls.Element>.OnCollectionChanged	C#
0x9 in System.Collections.ObjectModel.ObservableCollection<Microsoft.Maui.Controls.Element>.OnCollectionChanged	C#
0x23 in System.Collections.ObjectModel.ObservableCollection<Microsoft.Maui.Controls.Element>.InsertItem	C#
0x22 in System.Collections.ObjectModel.Collection<Microsoft.Maui.Controls.Element>.Add	C#
0x48 in Microsoft.Maui.Controls.TemplateUtilities.OnContentChanged at /_/src/Controls/src/Core/TemplateUtilities.cs:68,6	C#
0x188 in Microsoft.Maui.Controls.BindableObject.SetValueActual at /_/src/Controls/src/Core/BindableObject.cs:644,5	C#
0x168 in Microsoft.Maui.Controls.BindableObject.SetValueCore at /_/src/Controls/src/Core/BindableObject.cs:569,5	C#
0x22F in Microsoft.Maui.Controls.BindingExpression.ApplyCore at /_/src/Controls/src/Core/BindingExpression.cs:163,5	C#
0x4E in Microsoft.Maui.Controls.BindingExpression.Apply at /_/src/Controls/src/Core/BindingExpression.cs:58,5	C#
0x7 in Microsoft.Maui.Controls.BindingExpression.BindingExpressionPart.<PropertyChanged>b__50_0 at /_/src/Controls/src/Core/BindingExpression.cs:742,46	C#
0x6 in Microsoft.Maui.Dispatching.Dispatcher. at /_/src/Core/src/Dispatching/Dispatcher.Android.cs:24,24	C#
0xE in Java.Lang.Thread.RunnableImplementor.Run at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Java.Lang/Thread.cs:36,6	C#
0x8 in Java.Lang.IRunnableInvoker.n_Run at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net8.0/android-34/mcw/Java.Lang.IRunnable.cs:84,4	C#
0x8 in Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:22,5	C#

I've tried it both on my physical device and with the Standard Pixel 5 - API 34 Emulator. It happens both in Debug & Release mode.

I've wrapped every bit of code, that I suspect of being the culprit in try/catch statements. None of them seem to catch the exception. I've also tried to see if I can isolate the problem by observing the parallel stacks. Nothing gives me any usable information, where or what triggers the problem.

Does anyone have any idea what I could do or a tip how to proceed?

Now before anyone critizices the lack of further information: I know, I'm being very vague. But that's mainly because VS itself is extremely vague! I don't even know which part of my xaml or my code behind triggers the exception (if it even is my code that is the problem). I can't really post the code for all the custom views/converters/extensions I use on the page in question because it's a really big page and posting everything would totally break the scope and would take forever for me to rewrite it in a "anonymized" way.

I don't expect concrete solutions to my problem (although that would be awesome) but maybe someone can at least tell me how I could debug this more efficiently.

I tried the old clean, delete bin & obj and rebuild. I do it on the regular anyway because I know this is a common fix. Sadly didn't help.

Answer 1

Most likely different threads accessing same property, could be fixed with some kind of lock inside MAUI.

In your specific case you can wrap your code that modifies the value on both side (ViewModel and control with static properties) with MainThread.BeginInvokeOnMainThread.

This is just a workaround, that needs to be fixed inside MAUI, as the assumption was everyone would just use the main thread. Wrong.