[Bug]: 23.1.1 - Can't two-way convert between ViewModel and System.Object. To fix this, register a IBindingTypeConverter

[meta3ark]

Describe the bug 🐞

Converting project from 22.3.1 to 23.1.1 causes unexpected crash when using this.Bind().

System.ArgumentException
HResult=0x80070057
Message=Can't two-way convert between RxWpfApp.ItemViewModel and System.Object. To fix this, register a IBindingTypeConverter or call the version with the converter Func.
Source=ReactiveUI
StackTrace:
at ReactiveUI.PropertyBinderImplementation.Bind[TViewModel,TView,TVMProp,TVProp,TDontCare](TViewModel viewModel, TView view, Expression1 vmProperty, Expression1 viewProperty, IObservable1 signalViewUpdate, Object conversionHint, IBindingTypeConverter vmToViewConverterOverride, IBindingTypeConverter viewToVMConverterOverride, TriggerUpdate triggerUpdate) at ReactiveUI.PropertyBindingMixins.Bind[TViewModel,TView,TVMProp,TVProp](TView view, TViewModel viewModel, Expression1 vmProperty, Expression`1 viewProperty, Object conversionHint, IBindingTypeConverter vmToViewConverterOverride, IBindingTypeConverter viewToVMConverterOverride)
at RxWpfApp.MainWindow.<.ctor>b__0_0(CompositeDisposable d) in MainWindow.xaml.cs:line 18

Step to reproduce

1. Set break on all errors, otherwise project silently crashes.
2. Run attached project as 23.1.1. Exception thrown.
3. Switch the ReactiveUI package references back to 22.3.1. No exception thrown.

RxWpfApp.zip

Reproduction repository

No response

Expected behavior

No exception thrown for simple data template binding.

Screenshots 🖼️

n/a

IDE

No response

Operating system

Windows

Version

10

Device

No response

ReactiveUI Version

23.1.1

Additional information ℹ️

No response

[glennawatson]

This has been fixed in #4301.

The root cause was that the two-way binding validation required mutual type assignability -- both TVProp.IsAssignableFrom(TVMProp) and TVMProp.IsAssignableFrom(TVProp). This fails for common cases like binding a string view model property to an object view property, since string is assignable to object but not vice versa.

The fix relaxes the check to one-directional assignability (using || instead of &&). The existing fallback conversion logic already handles the direction that is not directly assignable.

This should be available in the next release.