public NetworkDiagram GetNetworkDiagram( 
   string name
)

using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
{
  // get all the diagrams
  IReadOnlyList<NetworkDiagram> diagrams = diagramManager.GetNetworkDiagrams();

  // get a diagram by name
  NetworkDiagram diagram = diagramManager.GetNetworkDiagram(diagrameName);

  // get diagrams by extent
  diagrams = diagramManager.GetNetworkDiagrams(extentOfInterest);

  // get diagrams from a set of utility network feature GlobalIDs
  diagrams = diagramManager.GetNetworkDiagrams(globalIDs);

  // get diagrams from a set of utility network feature GlobalIDs within an extent
  diagrams = diagramManager.GetNetworkDiagrams(extentOfInterest, globalIDs);
}

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);
  }
}