[Bug]: Inconsistent `BindCommand` behaviour when reusing a View with a new ViewModel

[xackus]

Describe the bug 🐞

When using BindCommand, the ViewModel passed to the withParameter expression can be different from the ViewModel the command is invoked on.

Step to reproduce

1. Use a ReactiveUI.Winforms.ViewModelControlHost with CacheViews = true.
2. Set a ViewModel.
3. Set a new ViewModel of the same type.
4. The command parameter is computed from the old ViewModel.

Reproduction repository

https://github.com/xackus/ReactiveUiBindCommandRepro

Expected behavior

The ViewModel passed to the withParameter expression is the same as the ViewModel the command is invoked on.

Screenshots 🖼️

No response

IDE

Visual Studio 2022

Operating system

Windows

Version

11

Device

PC

ReactiveUI Version

20.1.63

Additional information ℹ️

BindCommand internally uses Reflection.ViewModelWhenAnyValue for the subscription, which despite taking a ViewModel parameter, does not use it.
The only place where the ViewModel passed to BindCommand is actually used is when invoking the withParameter expression.

I find it very confusing that the Bind*(View, ViewModel, ...) family of functions seems to sometimes use the ViewModel that was passed in, and sometimes instead extracts the ViewModel from the View.
In my case I need the second behavior, because I reuse the View when possible. But then why pass in the ViewModel at all?
I haven't been using ReactiveUI for very long. Was the approach changed here some time ago?

[xackus]

I think I found another bug while researching this issue:

ReactiveUI/src/ReactiveUI/Bindings/Command/CommandBinder.cs

Lines 76 to 85 in 340e7b2

	public static IReactiveBinding<TView, TProp> BindCommand<TView, TViewModel, TProp, TControl>(
	this TView view,
	TViewModel? viewModel,
	Expression<Func<TViewModel, TProp?>> propertyName,
	Expression<Func<TView, TControl>> controlName,
	string? toEvent = null)
	where TView : class, IViewFor
	where TViewModel : class
	where TProp : ICommand =>
	_binderImplementation.BindCommand(viewModel, view, propertyName, controlName, toEvent);

Idk how this passes the type checker, but toEvent seems to be forwarded as the withParameter parameter.

[anaisbetts]

which despite taking a ViewModel parameter, does not use it.

The reason it does this is because it lets the C# compiler infer the TViewModel type for Expression<Func<TViewModel, TProp?>> propertyName, otherwise you would have to explicitly specify it and that'd be Annoying. The reason we specify a Func to get the ViewModel usually, is so that in the View, you can Replace the ViewModel and have everything still work

[xackus]

Makes sense! Thanks for the explanation.
I don't think this is mentioned anywhere in the docs. It definitely confused me. Maybe I'll make a PR later.