Invalid AXAML file compiles but gives misleading runtime errors

[astroman133]

I have been troubleshooting an issue for several hours. I decided to re-create the issue in a demo app and in the process I was able to see and correct the issue.

The symptoms are that the demo app compiles successfully, but at runtime gives the following errors:

[Binding]An error occurred binding 'ItemsSource' to 'People' at 'People': 'Items collection must be empty before using ItemsSource.' (ListBox #45083904)
[Binding]An error occurred binding 'ItemsSource' to 'People' at 'People': 'Items collection must be empty before using ItemsSource.' (ListBox #45083904)
Exception thrown: 'System.InvalidCastException' in System.Private.CoreLib.dll
[Binding]An error occurred binding 'Text' to 'FirstName' at 'FirstName': 'Unable to cast object of type 'System.String' to type 'TestListboxApp.Models.Person'.' (TextBlock #32167247)
Exception thrown: 'System.InvalidCastException' in System.Private.CoreLib.dll
[Binding]An error occurred binding 'Text' to 'LastName' at 'LastName': 'Unable to cast object of type 'System.String' to type 'TestListboxApp.Models.Person'.' (TextBlock #15521030)

I am using MVVM and binding to the ItemsSource property of the ListBox. I am not using the Items property at all. Also, the reported binding errors on the FirstName and LastName properties are misleading.

I was able to resolve the issue by deleting an errant character in the MainWindow.axaml file. It is on line 37 after the closing ItemTemplate tag.

The minimal project that re-creates the issue is attached. It was created with Visual Studio 2026 using Avalonia 11.3.11

TestListboxApp.zip