public bool QueryDiagramEdgeElement {get; set;}

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;
}

public void DiagramElementQueryResultAndNetworkDiagramSubsetClasses(Geodatabase geodatabase, DiagramManager diagramManager, string diagramName)
{
  // Retrieve a diagram
  using (NetworkDiagram diagramTest = diagramManager.GetNetworkDiagram(diagramName))
  {
    // Create a DiagramElementQueryByElementTypes query object to get the diagram elements we want to work with
    DiagramElementQueryByElementTypes query = new DiagramElementQueryByElementTypes();
    query.QueryDiagramJunctionElement = true;
    query.QueryDiagramEdgeElement = true;
    query.QueryDiagramContainerElement = true;

    // Retrieve those diagram elements
    DiagramElementQueryResult elements = diagramTest.QueryDiagramElements(query);

    // Create a NetworkDiagramSubset object to edit this set of diagram elements
    NetworkDiagramSubset subset = new NetworkDiagramSubset();
    subset.DiagramJunctionElements = elements.DiagramJunctionElements;
    subset.DiagramEdgeElements = elements.DiagramEdgeElements;
    subset.DiagramContainerElements = elements.DiagramContainerElements;

    // Edit the shapes of the diagram elements - left as an exercise for the student
    TranslateDiagramElements(subset);

    // Save the new layout of the diagram elements
    diagramTest.SaveLayout(subset, true);
  }
}