BooleanToVisibilityConverter.cs

public class BooleanToVisibilityConverter : MarkupExtension, IValueConverter
{
    public Visibility WhenTrue { get; set; }

    public Visibility WhenFalse { get; set; }

    public Visibility WhenNull { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider) => this;

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var boolean = value as bool?;
        if (boolean == null)
            return this.WhenNull;
        return boolean.Value ? this.WhenTrue : this.WhenFalse;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotSupportedException($"{nameof(NullableBoolConverter)} only supports one way bindings.");
    }
}