public sealed class LayoutProjectItem : ArcGIS.Desktop.Internal.Core.ProjectItem, ArcGIS.Desktop.Core.IMetadata, ArcGIS.Desktop.Core.IProjectItem, System.ComponentModel.INotifyPropertyChanged  

LayoutProjectItem layoutProjItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals("myLayout"));

//Gets all the layouts in the current project
var projectLayouts = Project.Current.GetItems<LayoutProjectItem>();
foreach (var layoutItem in projectLayouts)
{
  //Do Something with the layout
}

//GetItems searches project content
var map = Project.Current.GetItems<MapProjectItem>().FirstOrDefault(m => m.Name == "Map1");
var layout = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(m => m.Name == "Layout1");
var folders = Project.Current.GetItems<FolderConnectionProjectItem>();
var style = Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 3D");

//Find item uses a catalog path. The path can be to a file or dataset
var fcPath = @"C:\Pro\CommunitySampleData\Interacting with Maps\Interacting with Maps.gdb\Crimes";
var pdfPath = @"C:\Temp\Layout1.pdf";
var imgPath = @"C:\Temp\AddinDesktop16.png";

var fc = Project.Current.FindItem(fcPath);
var pdf = Project.Current.FindItem(pdfPath);
var img = Project.Current.FindItem(imgPath);

//This example references a graphic element on a layout and sets its Transparency property (which is not available in the managed API)
//by accessing the element's CIMGraphic.

//Added references
using ArcGIS.Core.CIM;                             //CIM
using ArcGIS.Desktop.Core;                         //Project
using ArcGIS.Desktop.Layouts;                      //Layout class
using ArcGIS.Desktop.Framework.Threading.Tasks;    //QueuedTask

public class GraphicElementExample1
{
  public static Task<bool> UpdateElementTransparencyAsync(string LayoutName, string ElementName, int TransValue)
  {
    //Reference a layoutitem in a project by name
    LayoutProjectItem layoutItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals(LayoutName));
    if (layoutItem == null)
      return Task.FromResult(false);

    return QueuedTask.Run<bool>(() =>
    {
      //Reference and load the layout associated with the layout item
      Layout lyt = layoutItem.GetLayout();

      //Reference a element by name
      GraphicElement graElm = lyt.FindElement(ElementName) as GraphicElement;
      if (graElm == null)
        return false;

      //Modify the Transparency property that exists only in the CIMGraphic class.
      CIMGraphic CIMGra = graElm.GetGraphic() as CIMGraphic;
      CIMGra.Transparency = TransValue;             //e.g., TransValue = 50
      graElm.SetGraphic(CIMGra);

      return true;
    });
  }
}

//Reference the layout associated with a layout project item.

LayoutProjectItem layoutItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals("Layout Name"));
Layout layout = await QueuedTask.Run(() => layoutItem.GetLayout());  //Perform on the worker thread

//Open the layout properties dialog.

//Get the layout associated with a layout project item
LayoutProjectItem lytItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals("Layout Name"));
Layout lyt = await QueuedTask.Run(() => lytItem.GetLayout());  //Worker thread

//Open the properties dialog
lyt.ShowProperties();  //GUI thread

//Report the event args when a layout is added.

//At 2.x - ArcGIS.Desktop.Layouts.Events.LayoutAddedEvent.Subscribe((args) =>
//{
//  System.Windows.MessageBox.Show("LayoutAddedEvent:" +
//    Environment.NewLine +
//    "   arg Layout: " + args.Layout.Name);
//});

//Use ProjectItemsChangedEvent at 3.x
ArcGIS.Desktop.Core.Events.ProjectItemsChangedEvent.Subscribe((args) => {
  //Layout added. Layout removed would be NotifyCollectionChangedAction.Remove
  if (args.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add &&
      args.ProjectItem is LayoutProjectItem layoutProjectItem)
  {
    var layout_name = layoutProjectItem.Name; 
    var layout = layoutProjectItem.GetLayout();
    System.Diagnostics.Debug.WriteLine($"Layout Added: {layout_name}");
  }
});

//Report the event args when a layout is removed.

//At 2.x - ArcGIS.Desktop.Layouts.Events.LayoutRemovedEvent.Subscribe((args) =>
//{
//  System.Windows.MessageBox.Show("LayoutViewEvent:" +
//  Environment.NewLine +
//  "   arg Layout: " + args.Layout.Name);
//});

//Use ProjectItemsChangedEvent at 3.x
ArcGIS.Desktop.Core.Events.ProjectItemsChangedEvent.Subscribe((args) => {
  //Layout added. Layout removed would be NotifyCollectionChangedAction.Remove
  if (args.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Remove &&
      args.ProjectItem is LayoutProjectItem layoutProjectItem)
  {
    var layout_name = layoutProjectItem.Name;
    System.Diagnostics.Debug.WriteLine($"Layout Removed: {layout_name}");
  }
});

//Report the event args when a layout is about to be removed.

//At 2.x - ArcGIS.Desktop.Layouts.Events.LayoutRemovingEvent.Subscribe((args) =>
//{
//  if (args.LayoutPath == "CIMPATH=layout.xml")
//  {
//    args.Cancel = true;
//  }
//  return Task.FromResult(0);
//});

//At 3.x use ProjectItemRemovingEvent
ArcGIS.Desktop.Core.Events.ProjectItemRemovingEvent.Subscribe((args) =>
{
  var layoutItems = args.ProjectItems.ToList().OfType<LayoutProjectItem>() ?? new List<LayoutProjectItem>();
  var layoutName = "DontDeleteThisOne";
  foreach(var layoutItem in layoutItems)
  {
    if (layoutItem.Name == layoutName)
    {
      args.Cancel = true;//Cancel the remove
      break;
    }
  }
  return Task.FromResult(0);
});

//This example deletes a layout in a project after finding it by name.

//Added references
using ArcGIS.Desktop.Core; 
using ArcGIS.Desktop.Layouts;

public class DeleteLayoutExample
{
  public static Task<bool> DeleteLayoutAsync(string LayoutName)
  {
    //Reference a layoutitem in a project by name
    LayoutProjectItem layoutItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals(LayoutName));

    //Check for layoutItem
    if (layoutItem == null)
      return Task.FromResult<bool>(false);

    //Delete the layout from the project
    return Task.FromResult<bool>(Project.Current.RemoveItem(layoutItem));
  }
}

//Find and close all layout panes associated with a specific layout.

LayoutProjectItem findLytItem = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(item => item.Name.Equals("Layout"));
Layout findLyt = await QueuedTask.Run(() => findLytItem.GetLayout());  //Perform on the worker thread

var panes = ProApp.Panes.FindLayoutPanes(findLyt);
foreach (Pane pane in panes)
{
  ProApp.Panes.CloseLayoutPanes(findLyt.URI);  //Close the pane
}

//Reference layout project items and their associated layout.
//A layout project item is an item that appears in the Layouts
//folder in the Catalog pane.

//Reference all the layout project items
IEnumerable<LayoutProjectItem> layouts = 
                     Project.Current.GetItems<LayoutProjectItem>();

//Or reference a specific layout project item by name
LayoutProjectItem layoutItem = Project.Current.GetItems<LayoutProjectItem>()
                           .FirstOrDefault(item => item.Name.Equals("MyLayout"));

//Open a layout project item in a new view.
//A layout project item may exist but it may not be open in a view. 

//Reference a layout project item by name
LayoutProjectItem someLytItem = Project.Current.GetItems<LayoutProjectItem>()
                          .FirstOrDefault(item => item.Name.Equals("MyLayout"));

//Get the layout associated with the layout project item
Layout layout = await QueuedTask.Run(() => someLytItem.GetLayout());  //Worker thread

//Create the new pane - call on UI
ILayoutPane iNewLayoutPane = await ProApp.Panes.CreateLayoutPaneAsync(layout); //GUI thread