public IReadOnlyList<DiagramContainerElement> DiagramContainerElements {get;}

{
  // Get the diagram elements whose extent intersects the active map extent
  static 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. 

    IReadOnlyList<DiagramContainerElement> containerElements = result.DiagramContainerElements;

    // In this case result.DiagramJunctionElements and result.DiagramEdgeElements will be empty
    // because elementQuery.QueryDiagramEdgeElement and elementQuery.QueryDiagramContainerElement are set to 'false'
    IReadOnlyList<DiagramJunctionElement> junctionElements = result.DiagramJunctionElements;

    IReadOnlyList<DiagramEdgeElement> edgeElements = result.DiagramEdgeElements;
  }
}