Validation for ReactiveUI based solutions, functioning in a reactive way. ReactiveUI.Validation was originally developed by @jcmm33 as Vistian.Reactive.Validation, and then refactored and updated by Àlex Martínez Morón and the ReactiveUI Core Team. ReactiveUI.Validation supports all platforms, including .NET Framework, .NET Standard, .NET Core, Maui.

Install the following package into your class library and into a platform-specific project.

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

• For ViewModels which need validation, implement IValidatableViewModel.
• Add validation rules to the ViewModel using the ValidationRule extension methods.
• Bind to the validation rules in the View via BindValidation or INotifyDataErrorInfo.

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.

using ReactiveUI.Validation.Extensions;

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.

using ReactiveUI.Validation.Extensions;

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.

There are extensions methods for Xamarin.Android and its Material design control TextInputLayout. These extensions use internally the Error property from the TextInputLayout control, allowing you to implement a fully native behavior to showing validation errors. To use these extensions you must import ReactiveUI.Validation.Extensions and install either ReactiveUI.Validation.AndroidSupport or ReactiveUI.Validation.AndroidX:

dotnet add package ReactiveUI.Validation.AndroidX

Note In ReactiveUI.Validation 1.7 and lower, the Android-specific extensions are available in the main package targeting MonoAndroid90, and you don't need to install ReactiveUI.Validation.AndroidSupport. In ReactiveUI.Validation 1.7 and lower, add using ReactiveUI.Validation.Platforms.Android instead of using ReactiveUI.Validation.Extensions.

// This using directive makes Android-specific extensions available.
// Make sure to install either the ReactiveUI.Validation.AndroidSupport
// package or the ReactiveUI.Validation.AndroidX package.
using ReactiveUI.Validation.Extensions;

public class SignUpActivity : ReactiveAppCompatActivity<SignUpViewModel>
{
    // The Android native text boxes declared in an .axml file.
    public TextInputEditText Password { get; set; }
    public TextInputEditText ConfirmPassword { get; set; }

    // The layouts wrapping the text boxes declared in an .axml file.
    public TextInputLayout PasswordField { get; set; }
    public TextInputLayout ConfirmPasswordField { get; set; }

    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.Main);

        // The WireUpControls method is a magic ReactiveUI utility method for Android, see:
        // https://www.reactiveui.net/docs/handbook/data-binding/xamarin-android/wire-up-controls
        this.WireUpControls();
        this.Bind(ViewModel, x => x.Password, x => x.Password.Text);
        this.Bind(ViewModel, x => x.ConfirmPassword, x => x.ConfirmPassword.Text);

        // Bind any validations which reference the Password property 
        // to the Error property of the TextInputLayout control.
        this.BindValidation(ViewModel, x => x.Password, PasswordField);
        this.BindValidation(ViewModel, x => x.ConfirmPassword, ConfirmPasswordField);
    }
}

For those platforms that support the INotifyDataErrorInfo interface, ReactiveUI.Validation provides a helper base class named ReactiveValidationObject. The helper class implements both the IValidatableViewModel interface and the INotifyDataErrorInfo interface. It listens to any changes in the ValidationContext and invokes INotifyDataErrorInfo events.

using ReactiveUI.Validation.Extensions;

public class SampleViewModel : ReactiveValidationObject
{
    public SampleViewModel()
    {
        this.ValidationRule(
            viewModel => viewModel.Name, 
            name => !string.IsNullOrWhiteSpace(name),
            "Name shouldn't be null or white space.");
    }

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

Note Keep in mind that INotifyDataErrorInfo is only supported via XAML binding. ReactiveUI binding doesn't use the inbuilt classes of WPF.

When using a ValidationRule overload that accepts an observable, please remember to supply the property which the validation rule is targeting as the first argument. Otherwise it is not possible for INotifyDataErrorInfo to conclude which property the error message is for.

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

You can pass an instance of IValidationTextFormatter<T> to a call to BindValidation if you'd like to override the default SingleLineFormatter used in the validation library. The SingleLineFormatter accepts a separator char and uses whitespace by default, so the code snippet below shows how to use a non-default separator char:

// This formatter is based on the default SingleLineFormatter but uses a custom separator char.
var formatter = new SingleLineFormatter(Environment.NewLine);
this.BindValidation(ViewModel, x => x.ErrorLabel.Text, formatter)
    .DisposeWith(disposables);

The simplest possible custom IValidationTextFormatter<TOut> implementation may look like this one.

private class ConstFormatter : IValidationTextFormatter<string>
{
    private readonly string _text;

    public ConstFormatter(string text = "The input is invalid.") => _text = text;

    public string Format(ValidationText validationText) => _text;
}

// This formatter is based on a custom IValidationTextFormatter implementation.
var formatter = new ConstFormatter("The input is invalid.");
this.BindValidation(ViewModel, x => x.ErrorLabel.Text, formatter)
    .DisposeWith(disposables);

If you'd like to override the IValidationTextFormatter<string> used in ReactiveUI.Validation by default, register an instance of IValidationTextFormatter<string> into Locator.CurrentMutable before your app starts. This could be useful in cases when your app needs localization and you wish to pass message keys instead of messages to ValidationRule calls.

// Register a singleton instance of IValidationTextFormatter<string> into Splat.Locator.
Locator.CurrentMutable.RegisterConstant(new CustomFormatter(), typeof(IValidationTextFormatter<string>));

In essence, ReactiveUI.Validation is a relatively simple model of the ValidationContext containing a list of IValidationComponent instances. An IValidationComponent provides an observable of IValidationState. Whenever validation state changes (either a transition of validity) or ValidationText changes, then a new value is pushed out.

1. Rules can be composed of single or multiple properties along with more generic Observables.
2. Validation text can encapsulate both valid and invalid states.
3. Binding can occur to either a View or an action.
4. Validation text can reference either the ViewModel or properties which comprise the validation rule e.g. include text entered as part of validation message.
5. Validation text output can be adjusted using custom formatters, not only allowing for single & multiline output but also for platforms like Android it should be possible to achieve richer renderings i.e. Bold/italics.

Please use GitHub issues for questions, comments, or bug reports.

ReactiveUI.Validation is developed under an OSI-approved open source license, making it freely usable and distributable, even for commercial use. We ❤ the people who are involved in this project, and we’d love to have you on board, especially if you are just getting started or have never contributed to open-source before.

So here's to you, lovely person who wants to join us — this is how you can support us:

• Responding to questions on StackOverflow
• Passing on knowledge and teaching the next generation of developers
• Submitting documentation updates where you see fit or lacking.
• Making contributions to the code base.

Code released under the MIT license.