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:

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

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.

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.