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passing a TextBlock or collection of inlines to a child UserControl in WPF
<myNS:InfoBox Text="Some Text"/>

public static readonly DependencyProperty TextProperty =
     DependencyProperty.Register("Text", typeof(string), typeof(InfoBox),
     new UIPropertyMetadata(null,ValueChanged));

private static void ValueChanged(DependencyObject dpo,
                                     DependencyPropertyChangedEventArgs args)
    {
        ((InfoBox)dpo).TextBlock.Text = (string)args.NewValue;
    }

<myNS:InfoBox>
        <myNS:InfoBox.ReferenceBlock>
             <TextBlock>
                <Run Language="en-gb" Text="SampleSample"/><LineBreak/>
                <Run Language="en-gb"/><LineBreak/>
                <Run Language="en-gb" Text="MoreMoreMore"/>
             </TextBlock>
        <myNS:InfoBox.ReferenceBlock>
</myNS:InfoBox>

public static readonly DependencyProperty ReferenceBlockProperty =
        DependencyProperty.Register("ReferenceBlock", typeof(TextBlock),
        typeof(InfoBox), new UIPropertyMetadata(null, ReferenceBlockReceived));

[...]


     private static void ReferenceBlockReceived(DependencyObject dpo,
            DependencyPropertyChangedEventArgs args)
    {
        var textblock = (TextBlock)args.NewValue;
        if (textblock != null)
        {
            ((InfoBox)dpo).TextBlock.Inlines.Clear();
            ((InfoBox)dpo).TextBlock.Inlines.AddRange(textblock.Inlines);
        }
    }

<ControlTemplate TargetType="myNS:InfoBox">
    <ContentControl Content="{TemplateBinding ReferenceBlock}" />
</ControlTemplate>