Binding to a MarkupExtension that Returns a Binding

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Is it possible to bind properties to a MarkupExtension that returns a Binding? Yes! The MarkupExtension just needs to return a BindingExpression by returning binding.ProvideValue(serviceProvider):

    [MarkupExtensionReturnType(typeof(BindingExpression))]

    public class PhysicalLengthMarkupExtension : MarkupExtension

    {

        public override object ProvideValue(IServiceProvider serviceProvider)

        {

            Binding physicalLengthBinding = new Binding();

            physicalLengthBinding.Source = _dpiProvider;

            physicalLengthBinding.Path = new PropertyPath("DPI");

            physicalLengthBinding.Converter = _dpiConverter;

            physicalLengthBinding.ConverterParameter = _length;

            return physicalLengthBinding.ProvideValue(serviceProvider);

        }

    }

Which enables the ability to write

<TextBlock Width="{Markup:PhysicalLengthMarkupExtension 3cm}"/>

 

instead of the more verbose and redundant

        <TextBlock>

            <TextBlock.Width>

< Binding

                  Source="{StaticResource DpiProvider}"

                  Path="DPI"

                  Converter="{StaticResource DpiConverter}"

                  ConverterParameter="2in"

                   />

            </TextBlock.Width>

        </TextBlock>

 

(Btw, I know that WPF takes DPI into account when doing layout calculations but let’s just use it as an example here.)