Breaking Changes ReactiveUI.WPF 21.0.1 to 22.2.1

[Theo-bos]

Upgrading from Version 21.0.1 to 22.1.1 or 22.2.1, I stumbled on a strange problem.
Simple use of ReactiveCommand.Create() errors in an InvalidOperationException of "The calling thread cannot access this object because a different thread owns it"

This happens only on the first Command Assigned.
// DummyCommand = ReactiveCommand.Create(() => { CommandAction = 0; });
Button1Command = ReactiveCommand.Create(() => { CommandAction = 1; });
Button2Command = ReactiveCommand.Create(() => { CommandAction = 2; });
Button3Command = ReactiveCommand.Create(() => { CommandAction = 3; });

If the order of Assigning Commands is changed, always the first Command will error in an Exception.
Assigning a DummyCommand (which is never used) can be a workaround.

The Commands are declared like this:
public ReactiveCommand<Unit, Unit> Button1Command { get; set; }
public ReactiveCommand<Unit, Unit> Button2Command { get; set; }
public ReactiveCommand<Unit, Unit> Button3Command { get; set; }

Downgrading from 22.2.1 to 21.0.1 or older, and the problem is gone.

Very simple App, nothing but one window with a few Buttons.
VS 2022 and VS2026
.Net9 and .Net10

ReactiveTest - 2022.zip

Does someone experience this same issue and is there a solution?
Or am I missing something in this simple approach with Reactive Commands?

[glennawatson]

Check out

https://www.reactiveui.net/docs/getting-started/installation/windows-presentation-foundation.html

See if the builder and the windows version help you

[Theo-bos]

Thanks Glenn, this solved my problem, but...
I am confused about the function and versioning of RxAppBuilder.
The Documentation says it was introduced in version 21.0.1. The sample App shows no Errors with that version, without the RxAppBuilder statement. Only upgrading ReactiveUI.WPF to version 22.1.1 or above, introduced this error.
The code ran without issues in version 20.4.1 also. What did the RxAppBuilder replace on Initialization that was not there before, and why does it only break on 22.x.x.?

[glennawatson]

A big issue is AOT and trimming that is the trend going forward in .net

The init previously was using reflection to init the application. Now there is a concept of the app builder which doesn't use the reflection to discovery registrations and instead initialisation directly with a explicit builder.

[Theo-bos]

OK, that's a plus. But should my second and third Command (in my sample) then not also terminate with that error?
Or would the first Command-creation trigger further initialization in version 22.x.x?