no Java peer type found - debug maui android

The failure is raised in Java.Interop.JniPeerMembers (constructor and GetConstructorsForType), which runs when a C# subclass of the Android NumberKeyListener is created without a registered Java peer. Start with the DecimalKeyListener class and its registration attributes, then check how the Java peer is looked up on device under debugging. Done means the listener constructs on a physical device while debugging, with a reproduction and a test or log that confirms it.

Android framework version

net10.0-android

Affected platform version

.NET 10.0.12

Description

I'm seeing the following crash whenever the following mapper code runs:

#if ANDROID
		// Remove when this issue is fixed
		// https://github.com/dotnet/maui/issues/39091
		EntryHandler.Mapper.AppendToMapping("CultureDecimal", (handler, view) =>
		{
			if (view.Keyboard != Keyboard.Numeric)
			{
				return;
			}

			// Accept both '.' and ',' so the decimal key is never silently
			// swallowed by the default locale-derived KeyListener.
			handler.PlatformView.KeyListener = new DecimalKeyListener();
		});
#endif

This appears to be only when debugging on a physical device, I see the following exception thrown:

[2026-10-16 09:43:03.568 +01:00] [FTL] [App] An unhandled exception occurred in the app domain (terminating: True)
System.NotSupportedException: Cannot create instance of type 'Acme.Presentation.Controls.Handlers.DecimalKeyListener': no Java peer type found.
   at Java.Interop.JniPeerMembers.JniInstanceMethods..ctor(Type declaringType)
   at Java.Interop.JniPeerMembers.JniInstanceMethods.GetConstructorsForType(Type declaringType)
   at Java.Interop.JniPeerMembers.JniInstanceMethods.StartCreateInstance(String constructorSignature, Type declaringType, JniArgumentValue* parameters)
   at Android.Text.Method.NumberKeyListener..ctor()
   at Acme.Presentation.Controls.Handlers.DecimalKeyListener..ctor()
   at Acme.Presentation.Hosting.MauiAppBuilderExtension.<>c.<RegisterAcmeControls>b__0_3(IEntryHandler handler, IEntry view) in /Users/axemasta/Documents/Dev/Infotechnics/Acme/AcmeWeb/src/Acme.Presentation/Hosting/MauiAppBuilderExtension.cs:line 68
   at Microsoft.Maui.PropertyMapperExtensions.<>c__DisplayClass3_0`2[[Microsoft.Maui.IEntry, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Handlers.IEntryHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<AppendToMapping>b__0(IEntryHandler handler, IEntry view, Action`2 action) in /_/src/Core/src/PropertyMapperExtensions.cs:line 84
   at Microsoft.Maui.PropertyMapperExtensions.<>c__DisplayClass0_0`2[[Microsoft.Maui.IEntry, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Handlers.IEntryHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<ModifyMapping>g__newMethod|0(IEntryHandler handler, IEntry view) in /_/src/Core/src/PropertyMapperExtensions.cs:line 23
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0[[Microsoft.Maui.IEntry, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Handlers.IEntryHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<Add>b__0(IElementHandler h, IElement v) in /_/src/Core/src/PropertyMapper.cs:line 239
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView) in /_/src/Core/src/PropertyMapper.cs:line 126
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view) in /_/src/Core/src/Handlers/Element/ElementHandler.cs:line 96
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IEntry, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.MauiAppCompatEditText, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IView view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 53
   at Microsoft.Maui.Handlers.EntryHandler.SetVirtualView(IView view) in /_/src/Core/src/Handlers/Entry/EntryHandler.Android.cs:line 30
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IEntry, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.MauiAppCompatEditText, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IElement view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 56
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler) in /_/src/Controls/src/Core/Element/Element.cs:line 1097
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/Element/Element.cs:line 1039
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/VisualElement/VisualElement.cs:line 2262
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 96
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 127
   at Microsoft.Maui.Handlers.LayoutHandler.SetVirtualView(IView view) in /_/src/Core/src/Handlers/Layout/LayoutHandler.Android.cs:line 41
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.ILayout, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.LayoutViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IElement view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 56
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler) in /_/src/Controls/src/Core/Element/Element.cs:line 1097
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/Element/Element.cs:line 1039
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/VisualElement/VisualElement.cs:line 2262
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 96
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 127
   at Microsoft.Maui.Handlers.BorderHandler.UpdateContent(IBorderHandler handler) in /_/src/Core/src/Handlers/Border/BorderHandler.Android.cs:line 47
   at Microsoft.Maui.Handlers.BorderHandler.MapContent(IBorderHandler handler, IBorderView border) in /_/src/Core/src/Handlers/Border/BorderHandler.cs:line 240
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0[[Microsoft.Maui.IBorderView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Handlers.IBorderHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<Add>b__0(IElementHandler h, IElement v) in /_/src/Core/src/PropertyMapper.cs:line 239
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView) in /_/src/Core/src/PropertyMapper.cs:line 126
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view) in /_/src/Core/src/Handlers/Element/ElementHandler.cs:line 96
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IBorderView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.ContentViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IView view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 53
   at Microsoft.Maui.Handlers.BorderHandler.SetVirtualView(IView view) in /_/src/Core/src/Handlers/Border/BorderHandler.Android.cs:line 29
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IBorderView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.ContentViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IElement view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 56
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler) in /_/src/Controls/src/Core/Element/Element.cs:line 1097
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/Element/Element.cs:line 1039
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/VisualElement/VisualElement.cs:line 2262
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 96
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 127
   at Microsoft.Maui.Handlers.ContentViewHandler.UpdateContent(IContentViewHandler handler) in /_/src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs:line 43
   at Microsoft.Maui.Handlers.ContentViewHandler.MapContent(IContentViewHandler handler, IContentView page) in /_/src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs:line 52
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0[[Microsoft.Maui.IContentView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Handlers.IContentViewHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<Add>b__0(IElementHandler h, IElement v) in /_/src/Core/src/PropertyMapper.cs:line 239
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView) in /_/src/Core/src/PropertyMapper.cs:line 126
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view) in /_/src/Core/src/Handlers/Element/ElementHandler.cs:line 96
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IContentView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.ContentViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IView view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 53
   at Microsoft.Maui.Handlers.ContentViewHandler.SetVirtualView(IView view) in /_/src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs:line 26
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IContentView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.ContentViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IElement view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 56
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler) in /_/src/Controls/src/Core/Element/Element.cs:line 1097
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/Element/Element.cs:line 1039
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/VisualElement/VisualElement.cs:line 2262
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 96
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 127
   at Microsoft.Maui.Handlers.ContentViewHandler.UpdateContent(IContentViewHandler handler) in /_/src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs:line 43
   at Microsoft.Maui.Handlers.ContentViewHandler.MapContent(IContentViewHandler handler, IContentView page) in /_/src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs:line 52
   at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0[[Microsoft.Maui.IContentView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Handlers.IContentViewHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<Add>b__0(IElementHandler h, IElement v) in /_/src/Core/src/PropertyMapper.cs:line 239
   at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView) in /_/src/Core/src/PropertyMapper.cs:line 126
   at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view) in /_/src/Core/src/Handlers/Element/ElementHandler.cs:line 96
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IContentView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.ContentViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IView view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 53
   at Microsoft.Maui.Handlers.ContentViewHandler.SetVirtualView(IView view) in /_/src/Core/src/Handlers/ContentView/ContentViewHandler.Android.cs:line 26
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.IContentView, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.ContentViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IElement view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 56
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler) in /_/src/Controls/src/Core/Element/Element.cs:line 1097
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/Element/Element.cs:line 1039
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/VisualElement/VisualElement.cs:line 2262
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 96
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 127
   at Microsoft.Maui.Handlers.LayoutHandler.SetVirtualView(IView view) in /_/src/Core/src/Handlers/Layout/LayoutHandler.Android.cs:line 41
   at Microsoft.Maui.Handlers.ViewHandler`2[[Microsoft.Maui.ILayout, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Platform.LayoutViewGroup, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].SetVirtualView(IElement view) in /_/src/Core/src/Handlers/View/ViewHandlerOfT.cs:line 56
   at Microsoft.Maui.Controls.Element.SetHandler(IElementHandler newHandler) in /_/src/Controls/src/Core/Element/Element.cs:line 1097
   at Microsoft.Maui.Controls.Element.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/Element/Element.cs:line 1039
   at Microsoft.Maui.Controls.VisualElement.Microsoft.Maui.IElement.set_Handler(IElementHandler value) in /_/src/Controls/src/Core/VisualElement/VisualElement.cs:line 2262
   at Microsoft.Maui.Platform.ElementExtensions.ToHandler(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 96
   at Microsoft.Maui.Platform.ElementExtensions.ToPlatform(IElement view, IMauiContext context) in /_/src/Core/src/Platform/ElementExtensions.cs:line 127
   at Microsoft.Maui.Handlers.LayoutHandler.Add(IView child) in /_/src/Core/src/Handlers/Layout/LayoutHandler.Android.cs:line 52
   at Microsoft.Maui.Handlers.LayoutHandler.MapAdd(ILayoutHandler handler, ILayout layout, Object arg) in /_/src/Core/src/Handlers/Layout/LayoutHandler.cs:line 85
   at Microsoft.Maui.CommandMapper`2.<>c__DisplayClass6_0[[Microsoft.Maui.ILayout, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.ILayoutHandler, Microsoft.Maui, Version=10.0.0.0, Culture=neutral, PublicKeyToken=null]].<Add>b__0(IElementHandler h, IElement v, Object o) in /_/src/Core/src/CommandMapper.cs:line 105
   at Microsoft.Maui.CommandMapper.InvokeCore(String key, IElementHandler viewHandler, IElement virtualView, Object args) in /_/src/Core/src/CommandMapper.cs:line 33
   at Microsoft.Maui.CommandMapper.Invoke(IElementHandler viewHandler, IElement virtualView, String property, Object args) in /_/src/Core/src/CommandMapper.cs:line 51
   at Microsoft.Maui.Handlers.ElementHandler.Invoke(String command, Object args) in /_/src/Core/src/Handlers/Element/ElementHandler.cs:line 114
   at Microsoft.Maui.Controls.Layout.NotifyHandler(String action, Int32 index, IView view) in /_/src/Controls/src/Core/Layout/Layout.cs:line 360
   at Microsoft.Maui.Controls.Layout.OnAdd(Int32 index, IView view) in /_/src/Controls/src/Core/Layout/Layout.cs:line 316
   at Microsoft.Maui.Controls.Layout.Add(IView child) in /_/src/Controls/src/Core/Layout/Layout.cs:line 190
   at Acme.Presentation.FormEngine.Implementations.PhoneFormEngine.<>c__DisplayClass67_1.<RenderAdditionalColumn>b__2() in /Users/axemasta/Documents/Dev/Infotechnics/Acme/AcmeWeb/src/Acme.Presentation/FormEngine/Implementations/PhoneFormEngine.cs:line 1021
   at Microsoft.Maui.Dispatching.Dispatcher.<>c__DisplayClass3_0.<DispatchImplementation>b__0() in /_/src/Core/src/Dispatching/Dispatcher.Android.cs:line 23
   at Java.Lang.Thread.RunnableImplementor.Run()
   at Java.Lang.IRunnableInvoker.n_Run(IntPtr jnienv, IntPtr native__this)

The code for the decimal key listener is fairly uninteresting. Its there to workaround a bug in maui where decimal comma cultures don't accept inputs from the decimal key on the OSK.

public class DecimalKeyListener : NumberKeyListener
{
	private static readonly char[] AcceptedChars = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.', ',', '-'];
	private static readonly char[] SeparatorChars = ['.', ','];

	public override InputTypes InputType =>
		InputTypes.ClassNumber | InputTypes.NumberFlagDecimal | InputTypes.NumberFlagSigned;

	protected override char[] GetAcceptedChars()
	{
		return AcceptedChars;
	}

	public override ICharSequence? FilterFormatted(
		ICharSequence? source,
		int start,
		int end,
		ISpanned? dest,
		int dstart,
		int dend)
	{
		var filtered = base.FilterFormatted(source, start, end, dest, dstart, dend);

		var incoming = filtered?.ToString() ?? source?.SubSequenceFormatted(start, end)?.ToString() ?? string.Empty;

		if (incoming.IndexOfAny(SeparatorChars) < 0)
		{
			return filtered;
		}

		var existing = dest?.ToString() ?? string.Empty;

		// The text that will remain once the [dstart, dend) range is replaced.
		var remainder = dend > dstart && dend <= existing.Length
			? existing.Remove(dstart, dend - dstart)
			: existing;

		if (remainder.IndexOfAny(SeparatorChars) < 0)
		{
			return filtered; // No separator outside the edited range yet: allow it through.
		}

		// A separator already exists elsewhere in the text: strip any further separators
		// from the incoming text so only one can ever be present.
		var stripped = new string(incoming.Where(c => Array.IndexOf(SeparatorChars, c) < 0).ToArray());

		return new Java.Lang.String(stripped);
	}
}

The only thing of note is that it's subclassing DecimalKeyListener : NumberKeyListener which i suspect may be the issue. I've had a look on this repo and all issues seem to be related to the linker however in my case ONLY debug builds are affected...

In terms of solution architecture the only thing of note is that the mapper is registered from a class library, not the main maui app (this class lib references net10-android and sets <UseMaui>true</UserMaui>. However I have tried moving that code to the main maui app, nuking bin obj and I see the same crash

Tested in 2 versions of rider + vscode so doesn't look like an IDE issue.
Raising this issue despite the fact I can't reproduce in a fresh sample since its a big roadblock to me in my current project and might be an issue to people out there so we need a papertrail!

Steps to Reproduce

I've not been able to reproduce outside of my main app, however it is a huge issue for me when working locally

• Install the mapper code
• Add a numeric entry to the first page in your application
• Debug the app

Expected:
App runs

Actual:
App crashes

Did you find any workaround?

None, tried:

• Adding protected constructors with the Jni bits passed through
• Adding register attribute
• Embedding assemblies in sdk on debug builds

Relevant log output