public sealed class DiagramManager : ArcGIS.Core.CoreObjectsBase, System.IDisposable  

using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
{
  // Todo - do something
}

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 List<NetworkDiagram> GetInconsistentDiagrams(UtilityNetwork utilityNetwork)
{
  // Get the DiagramManager from the utility network

  using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
  {
    List<NetworkDiagram> myList = new List<NetworkDiagram>();

    // Loop through the network diagrams in the diagram manager

    foreach (NetworkDiagram diagram in diagramManager.GetNetworkDiagrams())
    {
      NetworkDiagramInfo diagramInfo = diagram.GetDiagramInfo();

      // If the diagram is not a system diagram and is in an inconsistent state, add it to our list

      if (!diagramInfo.IsSystem && diagram.GetConsistencyState() != NetworkDiagramConsistencyState.DiagramIsConsistent)
      {
        myList.Add(diagram);
      }
      else
      {
        diagram.Dispose(); // If we are not returning it we need to Dispose it
      }
    }

    return myList;
  }
}

public void RetrieveDiagramTemplates(UtilityNetwork utilityNetwork)
{
  using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
  {
    // get all templates
    IReadOnlyList<DiagramTemplate> templates = diagramManager.GetDiagramTemplates();

    // get a template by name
    DiagramTemplate template = diagramManager.GetDiagramTemplate(templateName);
  }
}

public void GetNetworkDiagramFromDiagramTemplates(UtilityNetwork utilityNetwork)
{
  using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
  {
    // get the first templates
    DiagramTemplate template = diagramManager.GetDiagramTemplates().FirstOrDefault();

    // get the network diagrams fromt he template
    IEnumerable<NetworkDiagram> diagrams = template.GetNetworkDiagrams();

    // or get a network diagram by name
    NetworkDiagram diagram = template.GetNetworkDiagram(diagrameName);
  }
}

public void CreateNetworkDiagram(UtilityNetwork utilityNetwork, IEnumerable<Guid> globalIDs)
{
  using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
  {
    // get the template
    DiagramTemplate template = diagramManager.GetDiagramTemplate(templateName);

    // create the diagram
    NetworkDiagram diagram = diagramManager.CreateNetworkDiagram(template, globalIDs);
  }
}

public void GetDiagramContent(UtilityNetwork utilityNetwork)
{
  using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
  {
    // get a diagram by name
    NetworkDiagram diagram = diagramManager.GetNetworkDiagram(templateName);

    string json_content = diagram.GetContent(true, true, true, true);
  }
}