Public Sub CenterRect( _
   ByVal bounds As Rect, _
   ByVal afterwards As Action _
) 

public void CenterRect( 
   Rect bounds,
   Action afterwards
)

Parameters

bounds

a Rect in model coordinates

afterwards

an Action to be performed after the animation is finished; may be null

The HorizontalContentAlignment and VerticalContentAlignment properties need to be "Stretch" if the DiagramPanel.DiagramBounds are smaller than the viewport. But in any case, the bounds might not be centered if the panel cannot be scrolled far enough.

If you want to try to center the selection in the viewport, call:

            myDiagram.Panel.CenterRect(myDiagram.Panel.ComputeBounds(myDiagram.SelectedParts), null);
            

Reference