public IReadOnlyList<DiagramContainerElement> DiagramContainerElements {get;}

public void GetDiagramElements(MapView mapView, NetworkDiagram networkDiagram)
{
  // Create a DiagramElementQueryByExtent to retrieve diagram element junctions whose extent
  // intersects the active map extent

  DiagramElementQueryByExtent elementQuery = new DiagramElementQueryByExtent();
  elementQuery.ExtentOfInterest = MapView.Active.Extent;
  elementQuery.AddContents = false;
  elementQuery.QueryDiagramJunctionElement = true;
  elementQuery.QueryDiagramEdgeElement = false;
  elementQuery.QueryDiagramContainerElement = false;

  // Use this DiagramElementQueryByExtent as an argument to the QueryDiagramElements method
  DiagramElementQueryResult result = networkDiagram.QueryDiagramElements(elementQuery);

  // get the container, junction, edge elements
  //    in this case result.DiagramJunctionElements and result.DiagramEdgeElements will be empty 
  //    since elementQuery.QueryDiagramEdgeElement and elementQuery.QueryDiagramContainerElement are set to false
  IReadOnlyList<DiagramContainerElement> containerElements = result.DiagramContainerElements;

  IReadOnlyList<DiagramJunctionElement> junctionElements = result.DiagramJunctionElements;

  IReadOnlyList<DiagramEdgeElement> edgeElements = result.DiagramEdgeElements;
}