public NetworkDiagram GetNetworkDiagram( 
   string name
)

{
  static void GetDiagram(UtilityNetwork utilityNetwork, IEnumerable<Guid> globalIDs)
  {
    Envelope extentOfInterest = MapView.Active.Map.GetDefaultExtent();

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

      // Get a diagram by name
      NetworkDiagram diagram = diagramManager.GetNetworkDiagram(_diagramName);

      // 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);
    }
  }
}

{
  // Change the layout of a network diagram by querying for all the diagram elements, editing their shapes,
  static void ChangeDiagramLayout(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;

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