To implement a select-only behavior in the Diagram, follow these steps:

Create a custom behavior class:

private class SelectBehavior : BehaviorBase
{
      public SelectBehavior(Diagram diagram)
            : base(diagram)
      {
      }

      protected override bool OnMouseDown(MouseButtonEventArgs e)
      {
            return base.OnMouseDown(e);
      }

      public override Cursor SetMouseCursor(Point point, out bool startInteraction)
      {
            startInteraction = false;
            return Cursors.Arrow;
	}

      public override InteractionState StartDraw(Point point)
      {
            return new InteractionState(Diagram.Selection, Action.Create, -1, point);
      }
} 

Assign an instance of this class as behavior of the diagram:

diagram.CustomBehavior = new SelectBehavior(diagram);
diagram.Behavior = Behavior.Custom; 

Finally handle the SelectionMoving, NodeModifying and LinkModifying events and cancel dragging in each of them:

e.CancelDrag(); 

Now you can switch back and forth between the default diagram behavior and the newly implemented behavior.

Regards,
Meppy