using-converter-markup-extension

# Using Converter Markup Extension

Combine `MarkupExtension` with `IValueConverter` for direct XAML usage without resource declarations.

## Why Markup Extension Converters?

| Aspect | StaticResource Converter | MarkupExtension Converter |
|--------|-------------------------|--------------------------|
| Declaration | Required in Resources | Not required |
| XAML Usage | `{StaticResource MyConverter}` | `{local:MyConverter}` |
| Singleton | Manual implementation | Built-in lazy singleton |
| Boilerplate | More | Less |

---

## Base Classes

### IValueConverter Base (.NET 7+)

```csharp
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)
    {
        return _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();
    }
}
```

### IMultiValueConverter Base

```csharp
namespace MyApp.Converters;

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

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return _converter.Value;
    }

    public abstract object? Convert(
        object?[] values,
        Type targetType,
        object? parameter,
        CultureInfo culture);

    public virtual object?[] ConvertBack(
        object? value,
        Type[] targetTypes,
        object? parameter,
        CultureInfo culture)
    {
        throw new NotSupportedExcepti