RelativeSource MarkupExtension

Article
03/17/2022

Specifies properties of a RelativeSource binding source, to be used within a Binding Markup Extension, or when setting the RelativeSource property of a Binding element established in XAML.

XAML Attribute Usage

<Binding RelativeSource="{RelativeSource modeEnumValue}" ... />

XAML Attribute Usage (nested within Binding extension)

<object property="{Binding RelativeSource={RelativeSource modeEnumValue} ...}" ... />

XAML Object Element Usage

<Binding>
  <Binding.RelativeSource>
    <RelativeSource Mode="modeEnumValue"/>
  </Binding.RelativeSource>
</Binding>

-or-

<Binding>
  <Binding.RelativeSource>
    <RelativeSource
      Mode="FindAncestor"
      AncestorType="{x:Type typeName}"
      AncestorLevel="intLevel"
    />
  </Binding.RelativeSource>
</Binding>

XAML Values

Value	Description
`modeEnumValue`	One of the following: - The string token `Self`; corresponds to a RelativeSource as created with its Mode property set to Self. - The string token `TemplatedParent`; corresponds to a RelativeSource as created with its Mode property set to TemplatedParent. - The string token `PreviousData`; corresponds to a RelativeSource as created with its Mode property set to PreviousData. - See below for information on `FindAncestor` mode.
`FindAncestor`	The string token `FindAncestor`. Using this token enters a mode whereby a `RelativeSource` specifies an ancestor type and optionally an ancestor level. This corresponds to a RelativeSource as created with its Mode property set to FindAncestor.
`typeName`	Required for `FindAncestor` mode. The name of a type, which fills the AncestorType property.
`intLevel`	Optional for `FindAncestor` mode. An ancestor level (evaluated towards the parent direction in the logical tree).

Remarks

{RelativeSource TemplatedParent} binding usages are a key technique that addresses a larger concept of the separation of a control's UI and a control's logic. This enables binding from within the template definition to the templated parent (the run time object instance where the template is applied). For this case, the TemplateBinding Markup Extension is in fact a shorthand for the following binding expression: {Binding RelativeSource={RelativeSource TemplatedParent}}. TemplateBinding or {RelativeSource TemplatedParent} usages are both only relevant within the XAML that defines a template. For more information, see TemplateBinding Markup Extension.

{RelativeSource FindAncestor} is mainly used in control templates or predictable self-contained UI compositions, for cases where a control is always expected to be in a visual tree of a certain ancestor type. For example, items of an items control might use FindAncestor usages to bind to properties of their items control parent ancestor. Or, elements that are part of control composition in a template can use FindAncestor bindings to the parent elements in that same composition structure.

In the object element syntax for FindAncestor mode shown in the XAML Syntax sections, the second object element syntax is used specifically for FindAncestor mode. FindAncestor mode requires an AncestorType value. You must set AncestorType as an attribute using an x:Type Markup Extension reference to the type of ancestor to look for. The AncestorType value is used when the binding request is processed at run-time.

For FindAncestor mode, the optional property AncestorLevel can help disambiguate the ancestor lookup in cases where there is possibly more than one ancestor of that type existing in the element tree.

For more information on how to use the FindAncestor mode, see RelativeSource.

{RelativeSource Self} is useful for scenarios where one property of an instance should depend on the value of another property of the same instance, and no general dependency property relationship (such as coercion) already exists between those two properties. Although it is rare that two properties exist on an object such that the values are literally identical (and are identically typed), you can also apply a Converter parameter to a binding that has {RelativeSource Self}, and use the converter to convert between source and target types. Another scenario for {RelativeSource Self} is as part of a MultiDataTrigger.

For example, the following XAML defines a Rectangle element such that no matter what value is entered for Width, the Rectangle is always a square: <Rectangle Width="200" Height="{Binding RelativeSource={RelativeSource Self}, Path=Width}" .../>

{RelativeSource PreviousData} is useful either in data templates, or in cases where bindings are using a collection as the data source. You can use {RelativeSource PreviousData} to highlight relationships between adjacent data items in the collection. A related technique is to establish a MultiBinding between the current and previous items in the data source, and use a converter on that binding to determine the difference between the two items and their properties.

In the following example, the first TextBlock in the items template displays the current number. The second TextBlock binding is a MultiBinding that nominally has two Binding constituents: the current record, and a binding that deliberately uses the previous data record by using {RelativeSource PreviousData}. Then, a converter on the MultiBinding calculates the difference and returns it to the binding.

<ListBox Name="fibolist">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">
            <TextBlock Text="{Binding}"/>
            <TextBlock>, difference = </TextBlock>
                <TextBlock>
                    <TextBlock.Text>
                        <MultiBinding Converter="{StaticResource DiffConverter}">
                            <Binding/>
                            <Binding RelativeSource="{RelativeSource PreviousData}"/>
                        </MultiBinding>
                    </TextBlock.Text>
                </TextBlock>
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Describing data binding as a concept is not covered here, see Data Binding Overview.

In the WPF XAML processor implementation, the handling for this markup extension is defined by the RelativeSource class.

RelativeSource is a markup extension. Markup extensions are typically implemented when there is a requirement to escape attribute values to be other than literal values or handler names, and the requirement is more global than just putting type converters on certain types or properties. All markup extensions in XAML use the { and } characters in their attribute syntax, which is the convention by which a XAML processor recognizes that a markup extension must process the attribute. For more information, see Markup Extensions and WPF XAML.

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