Source:Open Clip Art Using Under Public Domain Attribution
I got a control in style template at one of my custom controls, Like this:
<Style TargetType="{x:Type advgenControls:CollapsibleControl}" x:Key="LeftPanel">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type advgenControls:CollapsibleControl}">
<Grid>

<ContentPresenter x:Name="PART_ContentPresenter"/>
<ResizeGrip Grid.Row="1" HorizontalAlignment="Stretch" Width="Auto" Height="Auto" HorizontalContentAlignment="Right" Margin="0,0,2,2"/>
</Grid>


</ControlTemplate>
</Setter.Value>
</Setter>

</Style>

I just want to call that "PART_ContentPresenter". But that is xaml living in resources.
That is easy I can use GetTemplateChild("PART_ContentPresenter" ) and call at the OnApplyTemplate(). Please remember don't use it at OnInitialized(EventArgs e). At that time, styles has not loaded yet, you get a null object.:

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