wpfMarkup Extensions

Parameters

MethodDescription
ProvideValueMarkupExtension class has only one method that should be overridden, XAML parser then uses the value provided by this method to evaluate the result of markup extension.

Remarks

A markup extension can be implemented to provide values for properties in an attribute usage, properties in a property element usage, or both.

When used to provide an attribute value, the syntax that distinguishes a markup extension sequence to a XAML processor is the presence of the opening and closing curly braces ({ and }). The type of markup extension is then identified by the string token immediately following the opening curly brace.

When used in property element syntax, a markup extension is visually the same as any other element used to provide a property element value: a XAML element declaration that references the markup extension class as an element, enclosed within angle brackets (<>).

For more info visit https://msdn.microsoft.com/en-us/library/ms747254(v=vs.110).aspx

Markup Extension used with IValueConverter

One of the best uses for markup extensions is for easier usage of IValueConverter. In the sample below BoolToVisibilityConverter is a value converter but since it's instance independent it can be used without the normal hasles of a value converter with the help of markup extension. In XAML just use

Visibility="{Binding [BoolProperty], Converter={[xmlns]:BoolToVisibilityConverter}}"

and you can set item visibility to bool value.

public class BoolToVisibilityConverter : MarkupExtension, IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value is bool)
                return (bool)value ? Visibility.Visible : Visibility.Collapsed;
            else
                return Visibility.Collapsed;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value is Visibility)
            {
                if ((Visibility)value == Visibility.Visible)
                    return true;
                else
                    return false;
            }
            else
                return false;
        }

        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            return this;
        }
    }

XAML-Defined markup extensions

There are four predefined markup extensions in XAML:

x:Type supplies the Type object for the named type. This facility is used most frequently in styles and templates.

<object property="{x:Type prefix:typeNameValue}" .../>

x:Static produces static values. The values come from value-type code entities that are not directly the type of a target property's value, but can be evaluated to that type.

<object property="{x:Static prefix:typeName.staticMemberName}" .../>

x:Null specifies null as a value for a property and can be used either for attributes or property element values.

<object property="{x:Null}" .../>

x:Array provides support for the creation of general arrays in XAML syntax, for cases where the collection support provided by WPF base elements and control models is deliberately not used.

<x:Array Type="typeName">
  arrayContents
</x:Array>