wpf-rule-converter-patterns

name	wpf-rule-converter-patterns
description	WPF IValueConverter rules: MarkupExtension singleton, pure functions, null/UnsetValue handling, TemplateBinding.
user-invocable	false

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   Lazy<T> _converter = (() =>  T());

     => _converter.Value;

      ? Convert(? , Type targetType, ? parameter, CultureInfo culture);

      ? ConvertBack(? , Type targetType, ? parameter, CultureInfo culture)
        =>   NotSupportedException();
}

    : <>
{
      ? Convert(? , Type targetType, ? parameter, CultureInfo culture)
    {
         (   ||  == DependencyProperty.UnsetValue)
             Visibility.Collapsed;

         invert = parameter    ;
         (   b && (b ^ invert)) ? Visibility.Visible : Visibility.Collapsed;
    }
}