BindValidation throws System.NotSupportedException

[NaolShow]

Describe the bug

When calling this.BindValidation on a ReactiveUserControl, it throws the exception:
System.NotSupportedException : 'Index expressions are only supported with constants.'

Steps To Reproduce

Follow: https://www.reactiveui.net/docs/handbook/user-input-validation/
And at the end, the bindValidation will throw an error.

Expected behavior

It should not throw an error and just bind correctly

Code

ConnectView:

public partial class ConnectView : ReactiveUserControl<ConnectViewModel> {

        public ConnectView() {
            InitializeComponent();

            // Input validation
            this.WhenActivated(disposables => {

                this.BindValidation(ViewModel, viewModel => viewModel.Username, view => view.FindControl<TextBlock>("UsernameError").Text).DisposeWith(disposables);

            });

        }

        private void InitializeComponent() {
            AvaloniaXamlLoader.Load(this);
        }

    }

ConnectViewModel:

public class ConnectViewModel : ReactiveObject, IValidatableViewModel {

        public ValidationContext ValidationContext { get; } = new ValidationContext();

        [Reactive] public string Username { get; set; } = "";
        [Reactive] public string Password { get; set; } = "";

        public ConnectViewModel() {

            // Username cannot be empty
            this.ValidationRule(viewModel => viewModel.Username, username => !string.IsNullOrEmpty(username), "Username cannot be empty..");

        }

    }

And of course I have a TextBlock with the x:Name attribute with a value of "UsernameError".

Additional context
I am using Avalonia.
Thanks for your help!

[glennawatson]

this.BindValidation(ViewModel, viewModel => viewModel.Username, view => view.FindControl<TextBlock>("UsernameError").Text).DisposeWith(disposables);

You can't do complex methods in the expression. You have FindControl. Just point towards a property only.

[worldbeater]

E.g. if you modify your ConnectView as such:

public partial class ConnectView : ReactiveUserControl<ConnectViewModel> {
    public ConnectView() {
        InitializeComponent();
        this.WhenActivated(disposables => {
            // Note the change in the third argument passed to BindValidation ⬇️
            this.BindValidation(ViewModel, vm => vm.Username, v => v.UsernameError.Text)
                .DisposeWith(disposables);
        });
    }

    private TextBlock UsernameError => this.FindControl<TextBlock>("UsernameError");

    private void InitializeComponent() {
            AvaloniaXamlLoader.Load(this);
    }
}

Your example will work, as this change makes the view expression understandable to ReactiveUI.Validation. See https://docs.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression?view=net-5.0 and https://www.reactiveui.net/docs/handbook/data-binding/#types-of-bindings for more info regarding expressions and their role in ReactiveUI code-behind bindings.

A general recommendation is to always declare references to Avalonia controls as such:

private TextBlock UsernameError => this.FindControl<TextBlock>("UsernameError");

And the good news is that there's a library called Avalonia.NameGenerator that will generate such references automatically. See documentation and generated code examples for more info. Don't forget to remove the InitializeComponent method and to use the method generated by Avalonia.NameGenerator btw.

[NaolShow]

Thank you so much guys!

It's working when I just create a property that call the FindControl method.
And yes I've seen that there is Avalonia.NameGenerator package, and I was wanting to use it afterwards (but because it was not working with the FindControl I didn't check with the NameGenerator package).
Wasn't knowing that calling a method right here could break everything.

Thank you again! I close this.

[github-actions]

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