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 NotSupportedException();
    }
}

Example Converters

BoolToVisibilityConverter

csharp

public sealed class BoolToVisibilityConverter : ConverterMarkupExtension<BoolToVisibilityConverter>
{
    public override object? Convert(
        object? value,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        if (value is not bool boolValue)
            return Visibility.Collapsed;

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

XAML Usage:

xml

<Button Visibility="{Binding IsEnabled, Converter={local:BoolToVisibilityConverter}}"/>

<!-- With parameter -->
<Button Visibility="{Binding IsDisabled, Converter={local:BoolToVisibilityConverter}, ConverterParameter=Invert}"/>

NullToVisibilityConverter

csharp

public sealed class NullToVisibilityConverter : ConverterMarkupExtension<NullToVisibilityConverter>
{
    public override object? Convert(
        object? value,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        var isNull = value is null;
        var invert = parameter is "Invert";
        return (isNull ^ invert) ? Visibility.Collapsed : Visibility.Visible;
    }
}

StringFormatConverter

csharp

public sealed class StringFormatConverter : ConverterMarkupExtension<StringFormatConverter>
{
    public override object? Convert(
        object? value,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        if (parameter is not string format)
            return value?.ToString();

        return string.Format(culture, format, value);
    }
}

XAML Usage:

xml

<TextBlock Text="{Binding Price, Converter={local:StringFormatConverter}, ConverterParameter='{}{0:C}'}"/>

FullNameConverter (Multi)

csharp

public sealed class FullNameConverter : MultiConverterMarkupExtension<FullNameConverter>
{
    public override object? Convert(
        object?[] values,
        Type targetType,
        object? parameter,
        CultureInfo culture)
    {
        if (values.Length < 2)
            return string.Empty;

        var firstName = values[0]?.ToString() ?? string.Empty;
        var lastName = values[1]?.ToString() ?? string.Empty;

        return $"{firstName} {lastName}".Trim();
    }
}

XAML Usage:

xml

<TextBlock>
    <TextBlock.Text>
        <MultiBinding Converter="{local:FullNameConverter}">
            <Binding Path="FirstName"/>
            <Binding Path="LastName"/>
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

GlobalUsings.cs

csharp

global using System;
global using System.Globalization;
global using System.Windows;
global using System.Windows.Data;
global using System.Windows.Markup;

Migration from StaticResource

Before

xml

<Window.Resources>
    <local:BoolToVisibilityConverter x:Key="BoolToVisibility"/>
</Window.Resources>

<Button Visibility="{Binding IsVisible, Converter={StaticResource BoolToVisibility}}"/>

After

xml

<!-- No resource declaration needed -->
<Button Visibility="{Binding IsVisible, Converter={local:BoolToVisibilityConverter}}"/>

File Structure

code

MyApp/
├── Converters/
│   ├── ConverterMarkupExtension.cs      # Base class
│   ├── MultiConverterMarkupExtension.cs # Multi base class
│   ├── BoolToVisibilityConverter.cs
│   ├── NullToVisibilityConverter.cs
│   └── StringFormatConverter.cs

Checklist

• Inherit from ConverterMarkupExtension<T> or MultiConverterMarkupExtension<T>
• Class is sealed (no inheritance needed)
• Handle null input gracefully
• Return DependencyProperty.UnsetValue for invalid input if needed
• Use ConverterParameter for variations instead of multiple converters