Xamarin Forms IValueConverter: ошибка преобразования обработки

Для Xamarin.Forms для этого можно использовать IValidationRule.

Во-первых, создание класса, производного от интерфейса IValidationRule, для указания правил проверки.

 public interface IValidationRule<T>
{
    string ValidationMessage { get; set; }
    bool Check(T value);
}

 public class IsNotNullOrEmptyRule<T> : IValidationRule<T>
{
    public string ValidationMessage { get; set; }

    public bool Check(T value)
    {
        if (value == null)
        {
            return false;
        }

        var str = $"{value }";
        return !string.IsNullOrWhiteSpace(str);
    }
}

public class HasValidAgeRule<T> : IValidationRule<T>
{
    public string ValidationMessage { get; set; }

    public bool Check(T value)
    {
        if (value is DateTime bday)
        {
            DateTime today = DateTime.Today;
            int age = today.Year - bday.Year;
            return (age >= 18);
        }

        return false;
    }
}

Во-вторых, добавление правил проверки к свойству.

public interface IValidatable<T> : INotifyPropertyChanged
{
    List<IValidationRule<T>> Validations { get; }

    List<string> Errors { get; set; }

    bool Validate();

    bool IsValid { get; set; }
}

 public class ValidatableObject<T> : IValidatable<T>
{
    public event PropertyChangedEventHandler PropertyChanged;

    public List<IValidationRule<T>> Validations { get; } = new List<IValidationRule<T>>();

    public List<string> Errors { get; set; } = new List<string>();

    public bool CleanOnChange { get; set; } = true;

    T _value;
    public T Value
    {
        get => _value;
        set
        {
            _value = value;

            if (CleanOnChange)
                IsValid = true;
        }
    }

    public bool IsValid { get; set; } = true;

    public virtual bool Validate()
    {
        Errors.Clear();

        IEnumerable<string> errors = Validations.Where(v => !v.Check(Value))
            .Select(v => v.ValidationMessage);

        Errors = errors.ToList();
        IsValid = !Errors.Any();

        return this.IsValid;
    }
    public override string ToString()
    {
        return $"{Value}";
    }
}

 public class validationmodel: INotifyPropertyChanged
{
    public ValidatableObject<string> FirstName { get; set; } = new ValidatableObject<string>();
    public ValidatableObject<string> LastName { get; set; } = new ValidatableObject<string>();
    public ValidatableObject<DateTime> BirthDay { get; set; } = new ValidatableObject<DateTime>() { Value = DateTime.Now };
    public validationmodel()
    {

        FirstName.Value = null;
        
        
        AddValidationRules();
        AreFieldsValid();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public void AddValidationRules()
    {
        FirstName.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "First Name Required" });
        LastName.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "Last Name Required" });
        BirthDay.Validations.Add(new HasValidAgeRule<DateTime> { ValidationMessage = "You must be 18 years of age or older" });
    }

    bool AreFieldsValid()
    {
        bool isFirstNameValid = FirstName.Validate();
        bool isLastNameValid = LastName.Validate();
        bool isBirthDayValid = BirthDay.Validate();
        return isFirstNameValid && isLastNameValid && isBirthDayValid;
    }



 }

Выделение элемента управления, содержащего недопустимые данные:

public class FirstValidationErrorConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        ICollection<string> errors = value as ICollection<string>;
        return errors != null && errors.Count > 0 ? errors.ElementAt(0) : null;
    }

   

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

 public class InverseBoolConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (!(value is bool))
        {
            throw new InvalidOperationException("The target must be a boolean");
        }

        return !(bool)value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return null;
    }
}

public class BehaviorBase<T> : Behavior<T>
where T : BindableObject
{
    #region Properties
    public T AssociatedObject
    {
        get;
        private set;
    }
    #endregion
    #region NormalMethods
    private void OnBindingContextChanged(object sender, EventArgs e)
    {
        OnBindingContextChanged();
    }
    #endregion
    #region Overrides
    protected override void OnAttachedTo(T bindable)
    {
        base.OnAttachedTo(bindable);
        AssociatedObject = bindable;
        if (bindable.BindingContext != null)
        {
            BindingContext = bindable.BindingContext;
        }

        bindable.BindingContextChanged += OnBindingContextChanged;
    }
    protected override void OnDetachingFrom(T bindable)
    {
        base.OnDetachingFrom(bindable);
        bindable.BindingContextChanged -= OnBindingContextChanged;
        AssociatedObject = null;
    }
    protected override void OnBindingContextChanged()
    {
        base.OnBindingContextChanged();
        BindingContext = AssociatedObject.BindingContext;
    }
    #endregion
}

 public class EntryLineValidationBehaviour : BehaviorBase<Entry>
{
    #region StaticFields
    public static readonly BindableProperty IsValidProperty = BindableProperty.Create(nameof(IsValid), typeof(bool), typeof(EntryLineValidationBehaviour), true, BindingMode.Default, null, (bindable, oldValue, newValue) => OnIsValidChanged(bindable, newValue));
    #endregion
    #region Properties
    public bool IsValid
    {
        get
        {
            return (bool)GetValue(IsValidProperty);
        }
        set
        {
            SetValue(IsValidProperty, value);
        }
    }
    #endregion
    #region StaticMethods
    private static void OnIsValidChanged(BindableObject bindable, object newValue)
    {
        if (bindable is EntryLineValidationBehaviour IsValidBehavior &&
             newValue is bool IsValid)
        {
            IsValidBehavior.AssociatedObject.PlaceholderColor = IsValid ? Color.Default : Color.Red;
        }
    }

    #endregion
}

Главная страница.xaml:

<ContentPage
x:Class = "validationapp.MainPage"
xmlns = "http://xamarin.com/schemas/2014/forms"
xmlns:x = "http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:behaviour = "clr-namespace:validationapp.Behaviors"
xmlns:converter = "clr-namespace:validationapp.converters"
xmlns:local = "clr-namespace:validationapp">
<ContentPage.Resources>
   
    <converter:InverseBoolConverter x:Key = "InverseBoolConverter" />
    <converter:FirstValidationErrorConverter x:Key = "FirstValidationErrorConverter" />
    <Style x:Key = "ErrorTextStyle" TargetType = "Label">
        <Setter Property = "TextColor" Value = "Red" />
        <Setter Property = "FontSize" Value = "12" />
    </Style>
</ContentPage.Resources>
<StackLayout>
    <!--  First Name  -->
    <Entry Placeholder = "First Name" Text = "{Binding FirstName.Value}">
        <Entry.Behaviors>
            <behaviour:EntryLineValidationBehaviour IsValid = "{Binding FirstName.IsValid}" />
        </Entry.Behaviors>
    </Entry>

    <Label
        IsVisible = "{Binding FirstName.IsValid, Converter = {StaticResource InverseBoolConverter}}"
        Style = "{StaticResource ErrorTextStyle}"
        Text = "{Binding FirstName.Errors, Converter = {StaticResource FirstValidationErrorConverter}}" />
    <!--  /First Name  -->

    <!--  Last Name  -->
    <Entry Placeholder = "Last Name" Text = "{Binding LastName.Value}">
        <Entry.Behaviors>
            <behaviour:EntryLineValidationBehaviour IsValid = "{Binding LastName.IsValid}" />
        </Entry.Behaviors>
    </Entry>

    <Label
        IsVisible = "{Binding LastName.IsValid, Converter = {StaticResource InverseBoolConverter}}"
        Style = "{StaticResource ErrorTextStyle}"
        Text = "{Binding LastName.Errors, Converter = {StaticResource FirstValidationErrorConverter}}" />
    <!--  /Last Name  -->


    <!--  Birthday  -->
    <DatePicker Date = "{Binding BirthDay.Value}" />
    <Label
        IsVisible = "{Binding BirthDay.IsValid, Converter = {StaticResource InverseBoolConverter}}"
        Style = "{StaticResource ErrorTextStyle}"
        Text = "{Binding BirthDay.Errors, Converter = {StaticResource FirstValidationErrorConverter}}" />
    <!--  Birthday  -->
</StackLayout>

Вы также можете запускать проверку при изменении свойств для команды Entry.

Более подробная информация о валидации, пожалуйста, посмотрите:

https://learn.microsoft.com/en-us/xamarin/xamarin-forms/enterprise-application-patterns/validation