ReactiveUI itself offers a few powerful features allowing you to validate user input on fly. With WhenAnyValue, you can listen to view model property changes and control ReactiveCommand executability. When reactive command's CanExecute observable returns false, the control to which you bind that command stays disabled. The simplest validator looks as follows:

// Declare name validator as IObservable<bool> which emitts a new value when name changes.
var nameValid = this.WhenAnyValue(x => x.Name, name => !string.IsNullOrWhiteSpace(name));

// The reactive command will stay disabled while name is invalid.
var saveName = ReactiveCommand.CreateFromTask(_ => Save(this.Name), canExecute: nameValid);

This definitely works for simple scenarios, but for larger forms and more complex validations you definitely need to give ReactiveUI.Validation package a try.

ReactiveUI.Validation

This is the primary way we make validations. The package contains validation helpers for ReactiveUI-based solutions, functioning in a reactive way. ReactiveUI.Validation sources are available on GitHub. The package supports all platforms, including .NET Framework, .NET Standard, MonoAndroid, Tizen, UAP, Xamarin.iOS, Xamarin.Mac, Xamarin.TVOS. Install the following package into you class library and into a platform-specific project.

Platform	ReactiveUI Package	NuGet
Any Platform	ReactiveUI.Validation
AndroidX (Xamarin)	ReactiveUI.Validation.AndroidX
Xamarin.Android	ReactiveUI.Validation.AndroidSupport

Getting Started

1. Decorate existing ViewModel with IValidatableViewModel, which has a single member, ValidationContext. The ValidationContext contains all of the functionality surrounding the validation of the ViewModel. Most access to the specification of validation rules is performed through extension methods on the IValidatableViewModel interface. Then, add validation to the ViewModel.

public class SampleViewModel : ReactiveObject, IValidatableViewModel
{
    public SampleViewModel()
    {
        // Creates the validation for the Name property.
        this.ValidationRule(
            viewModel => viewModel.Name,
            name => !string.IsNullOrWhiteSpace(name),
            "You must specify a valid name");
    }

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

    private string _name;
    public string Name
    {
        get => _name;
        set => this.RaiseAndSetIfChanged(ref _name, value);
    }
}

For more complex validation scenarios there are several more overloads of the ValidationRule extension method that accept observables. These allow validation to occur asynchronously, and allows complex chains of observables to be combined to produce validation results. The simplest accepts an IObservable<bool> where the observed boolean indicates whether the ValidationRule is valid or not. The overload accepts a message which is used when the observable produces a false (invalid) result.

IObservable<bool> passwordsObservable =
    this.WhenAnyValue(
        x => x.Password,
        x => x.ConfirmPassword,
        (password, confirmation) => password == confirmation);

this.ValidationRule(
    vm => vm.ConfirmPassword,
    passwordsObservable,
    "Passwords must match.");

Any existing observables can be used to drive a ValidationRule using the extension method overload that accepts an arbitrary IObservable<TState> streams of events. The overload accepts a custom validation function that is supplied with the latest TState, and a custom error message function, responsible for formatting the latest TState object. The syntax for this looks as follows:

// IObservable<{ Password, Confirmation }>
var passwordsObservable =
    this.WhenAnyValue(
        x => x.Password,
        x => x.ConfirmPassword,
        (password, confirmation) =>
            new { Password = password, Confirmation = confirmation });

this.ValidationRule(
    vm => vm.ConfirmPassword,
    passwordsObservable,
    state => state.Password == state.Confirmation,
    state => $"Passwords must match: {state.Password} != {state.Confirmation}");

Note The function to extract a message (messageFunc) is only invoked if the function to establish validity (isValidFunc) returns false, otherwise the message is set to string.Empty.

Finally, you can directly supply an observable that streams any object (or struct) that implements IValidationState; or you can use the ValidationState base class which already implements the interface. As the resulting object is stored directly against the context without further transformation, this can be the most performant approach:

IObservable<IValidationState> usernameNotEmpty =
    this.WhenAnyValue(x => x.UserName)
        .Select(name => string.IsNullOrEmpty(name) 
            ? new ValidationState(false, "The username must not be empty")
            : ValidationState.Valid);

this.ValidationRule(vm => vm.UserName, usernameNotEmpty);

Note As a valid ValidationState does not really require a message, there is a singleton ValidationState.Valid property that you are encouraged to use to indicate a valid state whenever possible, to reduce memory allocations.

2. Add validation presentation to the View.

public class SampleView : ReactiveContentPage<SampleViewModel>
{
    public SampleView()
    {
        InitializeComponent();
        this.WhenActivated(disposables =>
        {
            this.Bind(ViewModel, vm => vm.Name, view => view.Name.Text)
                .DisposeWith(disposables);

            // Bind any validations that reference the Name property 
            // to the text of the NameError UI control.
            this.BindValidation(ViewModel, vm => vm.Name, view => view.NameError.Text)
                .DisposeWith(disposables);

            // Bind any validations attached to this particular view model
            // to the text of the FormErrors UI control.
            this.BindValidation(ViewModel, view => view.FormErrors.Text)
                .DisposeWith(disposables);
        });
    }
}

Note Name is an <Entry />, NameError is a <Label />, and FormErrors is a <Label /> as well. All these controls are from the Xamarin.Forms library.

Extended ReactiveUI.Validation Usage

ReactiveUI.Validation also supports INotifyDataErrorInfo validations used by XAML platforms, including WPF and Avalonia; binding to TextInputLayout controls from Xamarin.Android and AndroidX, custom formatters that could be used for localization. Head over to ReactiveUI.Validation GitHub page to learn more about extended usage of this ReactiveUI library:

• Example with Android Extensions
• INotifyDataErrorInfo Support
• Custom Formatters

Alternatives

You can also use Xamarin validations as explained in the David Britch's article. Other great tools for user input validation are FluentValidation and Sprache.