Attributes

Property
Orientation

Example

<dev:ElementGroup>
    <Button HorizontalAlignment="Stretch"
            Content="Button" />
    <Button HorizontalAlignment="Stretch"
            Content="Button" />
    <Button HorizontalAlignment="Stretch"
            Content="Button" />
    <Button HorizontalAlignment="Stretch"
            Content="Button" />
</dev:ElementGroup>

Demo

you can run demo and see this feature.