/// <summary>
        /// Setup a validation rule with a general observable based on <see cref="IValidationState"/>.
        /// </summary>
        /// <typeparam name="TViewModel">ViewModel type.</typeparam>
        /// <typeparam name="TValue">Validation observable type.</typeparam>
        /// <param name="viewModel">ViewModel instance.</param>
        /// <param name="validationObservable">Observable to define if the viewModel is valid or not.</param>
        /// <returns>Returns a <see cref="ValidationHelper"/> object.</returns>
        /// <remarks>
        /// It should be noted that the observable should provide an initial value, otherwise that can result
        /// in an inconsistent performance.
        /// </remarks>
        public static ValidationHelper ValidationRule<TViewModel, TValue>(
            this TViewModel viewModel,
            IObservable<TValue> validationObservable)
            where TViewModel : IReactiveObject, IValidatableViewModel
            where TValue : IValidationState
        {
            if (viewModel is null)
            {
                throw new ArgumentNullException(nameof(viewModel));
            }

            if (validationObservable is null)
            {
                throw new ArgumentNullException(nameof(validationObservable));
            }

            return viewModel.RegisterValidation(
                new ObservableValidation<TViewModel, bool>(
                    validationObservable.Select(s => s as IValidationState)));
        }

        /// <summary>
        /// Setup a validation rule with a general observable based on <see cref="IValidationState"/>.
        /// </summary>
        /// <typeparam name="TViewModel">ViewModel type.</typeparam>
        /// <typeparam name="TViewModelProp">ViewModel property type.</typeparam>
        /// <typeparam name="TValue">Validation observable type.</typeparam>
        /// <param name="viewModel">ViewModel instance.</param>
        /// <param name="viewModelProperty">ViewModel property referenced in viewModelObservableProperty.</param>
        /// <param name="validationObservable">Observable to define if the viewModel is valid or not.</param>
        /// <returns>Returns a <see cref="ValidationHelper"/> object.</returns>
        /// <remarks>
        /// It should be noted that the observable should provide an initial value, otherwise that can result
        /// in an inconsistent performance.
        /// </remarks>
        public static ValidationHelper ValidationRule<TViewModel, TViewModelProp, TValue>(
            this TViewModel viewModel,
            Expression<Func<TViewModel, TViewModelProp>> viewModelProperty,
            IObservable<TValue> validationObservable)
            where TViewModel : IReactiveObject, IValidatableViewModel
            where TValue : IValidationState
        {
            if (viewModel is null)
            {
                throw new ArgumentNullException(nameof(viewModel));
            }

            if (viewModelProperty is null)
            {
                throw new ArgumentNullException(nameof(viewModelProperty));
            }

            if (validationObservable is null)
            {
                throw new ArgumentNullException(nameof(validationObservable));
            }

            return viewModel.RegisterValidation(
                new ObservableValidation<TViewModel, bool, TViewModelProp>(
                    viewModelProperty, validationObservable.Select(v => v as IValidationState)));
        }

IObservable<ValidationState> observable = ...;
model.ValidationRule(observable.Select(v => v as IValidationState));