wpf-rule-converter-patterns

Converter Patterns

Standards for implementing IValueConverter in wpf-dev-pack projects.

MarkupExtension Singleton Pattern

Every converter is a MarkupExtension so it can be used directly in XAML without declaring a resource, and ProvideValue returns a single shared instance (converters are stateless, so a shared instance is safe and avoids ResourceDictionary clutter). This is the canonical converter pattern, matching the /wpf-dev-pack:make-wpf-converter scaffolder and the using-converter-markup-extension knowledge topic. Pick this one pattern — do NOT also expose a separate static Instance property.

Derive from a small base class (the scaffolder generates it once per project):

namespace MyApp.Converters;

public abstract class ConverterMarkupExtension<T> : MarkupExtension, IValueConverter
    where T : class, new()
{
    private static readonly Lazy<T> _converter = new(() => new T());

    public override object ProvideValue(IServiceProvider serviceProvider) => _converter.Value;

    public abstract object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture);

    public virtual object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
        => throw new NotSupportedException("ConvertBack is not supported.");
}

public sealed class BoolToVisibilityConverter : ConverterMarkupExtension<BoolToVisibilityConverter>
{
    public override object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        if (value is null || value == DependencyProperty.UnsetValue)
            return Visibility.Collapsed;

        var invert = parameter is "Invert" or "invert";
        return (value is bool b && (b ^ invert)) ? Visibility.Visible : Visibility.Collapsed;
    }
}

Usage in XAML (no ResourceDictionary entry required):

<TextBlock Visibility="{Binding IsActive,
               Converter={converters:BoolToVisibilityConverter}}" />