public sealed class StyleProjectItem : ArcGIS.Desktop.Internal.Core.ProjectItem, ArcGIS.Desktop.Core.IMetadata, ArcGIS.Desktop.Core.IProjectItem, System.ComponentModel.INotifyPropertyChanged, System.IComparable<StyleProjectItem>, System.IDisposable  

IEnumerable<StyleProjectItem> newStyleItemsContainer = null;
newStyleItemsContainer = Project.Current.GetItems<StyleProjectItem>();
foreach (var styleItem in newStyleItemsContainer)
{
  //Do Something with the style.
}

var container = Project.Current.GetItems<StyleProjectItem>();
StyleProjectItem testStyle = container.FirstOrDefault(style => (style.Name == "ArcGIS 3D"));
StyleItem cone = null;
if (testStyle != null)
  cone = testStyle.LookupItem(StyleItemType.PointSymbol, "Cone_Volume_3");

var fav_style_item = await QueuedTask.Run(() =>
{
  var containerStyle = Project.Current.GetProjectItemContainer("Style");
  return containerStyle.GetItems().OfType<StyleProjectItem>().First(item => item.TypeID == "personal_style");
});

//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);

//Create a scale bar for a specific map frame and assign a scale bar style item.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Reference a North Arrow in a style
  StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(item => item.Name == "ArcGIS 2D");
  ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars("Double Alternating Scale Bar 1")[0];

  //Build geometry
  Coordinate2D center = new Coordinate2D(7, 8);

  //Reference MF, create north arrow and add to layout 
  MapFrame mf = layout.FindElement("New Map Frame") as MapFrame;
  if (mf == null)
  {
    ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show("Map frame not found", "WARNING");
    return;
  }

  //At 2.x -
  //ScaleBar sbElm = LayoutElementFactory.Instance.CreateScaleBar(layout, center, mf, sbStyleItm);
  //sbElm.SetName("New Scale Bar");
  //sbElm.SetWidth(2);
  //sbElm.SetX(6);
  //sbElm.SetY(7.5); 

  var sbInfo = new ScaleBarInfo()
  {
    MapFrameName = mf.Name,
    ScaleBarStyleItem = sbStyleItm
  };
  var sbElm = ElementFactory.Instance.CreateMapSurroundElement(
                                      layout, center.ToMapPoint(), sbInfo, "New Scale Bar") as ScaleBar;
  sbElm.SetWidth(2);
  sbElm.SetX(6);
  sbElm.SetY(7.5);
});

//Create a north arrow for a specific map frame and assign a north arrow style item.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Reference a North Arrow in a style
  StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(item => item.Name == "ArcGIS 2D");
  NorthArrowStyleItem naStyleItm = stylePrjItm.SearchNorthArrows("ArcGIS North 10")[0];

  //Build geometry
  Coordinate2D center = new Coordinate2D(7, 5.5);

  //Reference MF, create north arrow and add to layout 
  MapFrame mf = layout.FindElement("New Map Frame") as MapFrame;
  if (mf == null)
  {
    ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show("Map frame not found", "WARNING");
    return;
  }
  //At 2.x -
  //NorthArrow arrowElm = LayoutElementFactory.Instance.CreateNorthArrow(layout, center, mf, naStyleItm);
  //arrowElm.SetName("New North Arrow");
  //arrowElm.SetHeight(1.75);
  //arrowElm.SetX(7);
  //arrowElm.SetY(6);

  var naInfo = new NorthArrowInfo()
  {
    MapFrameName = mf.Name,
    NorthArrowStyleItem = naStyleItm
  };

  var arrowElm = ElementFactory.Instance.CreateMapSurroundElement(
                            layout, center.ToMapPoint(), naInfo, "New North Arrow") as NorthArrow;
  arrowElm.SetHeight(1.75);
  arrowElm.SetX(7);
  arrowElm.SetY(6);
});

//Must be on QueuedTask.Run(() => { ...

//Build geometry
List<Coordinate2D> plCoords = new List<Coordinate2D>();
plCoords.Add(new Coordinate2D(1, 8.5));
plCoords.Add(new Coordinate2D(1.66, 9));
plCoords.Add(new Coordinate2D(2.33, 8.1));
plCoords.Add(new Coordinate2D(3, 8.5));
Polyline linePl = PolylineBuilderEx.CreatePolyline(plCoords);

//Reference a line symbol in a style
var ProjectStyles = Project.Current.GetItems<StyleProjectItem>();
StyleProjectItem style = ProjectStyles.First(x => x.Name == "ArcGIS 2D");
var symStyle = style.SearchSymbols(StyleItemType.LineSymbol, "Line with 2 Markers")[0];
CIMLineSymbol lineSym = symStyle.Symbol as CIMLineSymbol;
lineSym.SetSize(20);

//Set symbology, create and add element to layout
//CIMLineSymbol lineSym = SymbolFactory.Instance.ConstructLineSymbol(ColorFactory.Instance.BlueRGB, 4.0, SimpleLineStyle.Solid);
ElementFactory.Instance.CreateGraphicElement(
  container, linePl, lineSym, "New Line");

//Must be on QueuedTask.Run(() => { ...

//Build geometry
Coordinate2D coord2D = new Coordinate2D(2.0, 10.0);

//Reference a point symbol in a style
StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>()
         .FirstOrDefault(item => item.Name == "ArcGIS 2D");
SymbolStyleItem symStyleItm = stylePrjItm.SearchSymbols(
                      StyleItemType.PointSymbol, "City Hall")[0];
CIMPointSymbol pointSym = symStyleItm.Symbol as CIMPointSymbol;
pointSym.SetSize(50);

var elemInfo = new ElementInfo()
{
  CustomProperties = new List<CIMStringMap>() {
     new CIMStringMap() { Key = "Key1", Value = "Value1"},
     new CIMStringMap() { Key = "Key2", Value = "Value2"}
   },
  Anchor = Anchor.TopRightCorner,
  Rotation = 45.0
};

var graphic = GraphicFactory.Instance.CreateSimpleGraphic(
                              coord2D.ToMapPoint(), pointSym);

ElementFactory.Instance.CreateGraphicElement(
  container, graphic, "New Point", true, elemInfo);

//Create a simple 2D point graphic and apply an existing point style item as the symbology.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2D point geometry  
  Coordinate2D coord2D = new Coordinate2D(2.0, 10.0);

  //(optionally) Reference a point symbol in a style
  StyleProjectItem ptStylePrjItm = Project.Current.GetItems<StyleProjectItem>()
                                .FirstOrDefault(item => item.Name == "ArcGIS 2D");
  SymbolStyleItem ptSymStyleItm = ptStylePrjItm.SearchSymbols(
                                         StyleItemType.PointSymbol, "City Hall")[0];
  CIMPointSymbol pointSym = ptSymStyleItm.Symbol as CIMPointSymbol;
  pointSym.SetSize(50);

  //Set symbology, create and add element to layout

  //An alternative simple symbol is also commented out below.
  //This would eliminate the four optional lines of code above that
  //reference a style.

  //CIMPointSymbol pointSym = SymbolFactory.Instance.ConstructPointSymbol(
  //                  ColorFactory.Instance.RedRGB, 25.0, SimpleMarkerStyle.Star);  
  //At 2.x - GraphicElement ptElm =
  //                    LayoutElementFactory.Instance.CreatePointGraphicElement(
  //                                        layout, coord2D, pointSym);

  GraphicElement ptElm = ElementFactory.Instance.CreateGraphicElement(
                                 container, coord2D.ToMapPoint(), pointSym);
  ptElm.SetName("New Point");
});

//Create a simple 2D line graphic and apply an existing line
//style item as the symbology.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2d line geometry
  List<Coordinate2D> plCoords = new List<Coordinate2D>();
  plCoords.Add(new Coordinate2D(1, 8.5));
  plCoords.Add(new Coordinate2D(1.66, 9));
  plCoords.Add(new Coordinate2D(2.33, 8.1));
  plCoords.Add(new Coordinate2D(3, 8.5));
  //At 2.x - Polyline linePl = PolylineBuilder.CreatePolyline(plCoords);
  Polyline linePl = PolylineBuilderEx.CreatePolyline(plCoords);

  //(optionally) Reference a line symbol in a style
  StyleProjectItem lnStylePrjItm = Project.Current.GetItems<StyleProjectItem>()
                                .FirstOrDefault(item => item.Name == "ArcGIS 2D");
  SymbolStyleItem lnSymStyleItm = lnStylePrjItm.SearchSymbols(
                                StyleItemType.LineSymbol, "Line with 2 Markers")[0];
  CIMLineSymbol lineSym = lnSymStyleItm.Symbol as CIMLineSymbol;
  lineSym.SetSize(20);

  //Set symbology, create and add element to layout

  //An alternative simple symbol is also commented out below.
  //This would eliminate the four optional lines of code above that
  //reference a style.
  //
  //CIMLineSymbol lineSym = SymbolFactory.Instance.ConstructLineSymbol(
  //         ColorFactory.Instance.BlueRGB, 4.0, SimpleLineStyle.Solid);  
  //At 2.x - GraphicElement lineElm =
  //        LayoutElementFactory.Instance.CreateLineGraphicElement(
  //                                            layout, linePl, lineSym);

  GraphicElement lineElm = ElementFactory.Instance.CreateGraphicElement(
                                              container, linePl, lineSym);
  lineElm.SetName("New Line");
});

//Create a simple 2D rectangle graphic and apply simple fill and
//outline symbols.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2D envelope geometry
  Coordinate2D rec_ll = new Coordinate2D(1.0, 4.75);
  Coordinate2D rec_ur = new Coordinate2D(3.0, 5.75);
  //At 2.x - Envelope rec_env = EnvelopeBuilder.CreateEnvelope(rec_ll, rec_ur);
  Envelope rec_env = EnvelopeBuilderEx.CreateEnvelope(rec_ll, rec_ur);

  //Set symbology, create and add element to layout
  CIMStroke outline = SymbolFactory.Instance.ConstructStroke(
    ColorFactory.Instance.BlackRGB, 5.0, SimpleLineStyle.Solid);
  CIMPolygonSymbol polySym = SymbolFactory.Instance.ConstructPolygonSymbol(
    ColorFactory.Instance.GreenRGB, SimpleFillStyle.DiagonalCross, outline);

  //At 2.x - GraphicElement recElm =
  //           LayoutElementFactory.Instance.CreateRectangleGraphicElement(
  //                                                  layout, rec_env, polySym);
  //         recElm.SetName("New Rectangle");
  //
  GraphicElement recElm = ElementFactory.Instance.CreateGraphicElement(
    container, rec_env, polySym, "New Rectangle");
  //Or use Predefined shape
  GraphicElement recElm2 = ElementFactory.Instance.CreatePredefinedShapeGraphicElement(
                            container, PredefinedShape.Rectangle, rec_env, polySym, 
                            "New Rectangle2");
});

//Create a scale bar using a style.

//Search for a style project item by name
StyleProjectItem arcgis_2dStyle = Project.Current.GetItems<StyleProjectItem>()
                            .First(si => si.Name == "ArcGIS 2D");

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Reference the specific scale bar by name 
  ScaleBarStyleItem scaleBarItem = arcgis_2dStyle.SearchScaleBars(
                     "Double Alternating Scale Bar").FirstOrDefault();

  //Reference the map frame and define the location
  MapFrame myMapFrame = layout.FindElement("Map Frame") as MapFrame;
  Coordinate2D coord2D = new Coordinate2D(10.0, 7.0);

  //Construct the scale bar
  //At 2.x - LayoutElementFactory.Instance.CreateScaleBar(
  //             layout, coord2D, myMapFrame, scaleBarItem);
  var sbarInfo = new ScaleBarInfo()
  {
    MapFrameName = myMapFrame.Name,
    ScaleBarStyleItem = scaleBarItem
  };
  ElementFactory.Instance.CreateMapSurroundElement(
          layout, coord2D.ToMapPoint(), sbarInfo);
});

//Create a north arrow using a style.

//Search for a style project item by name
StyleProjectItem arcgis2dStyles = Project.Current.GetItems<StyleProjectItem>()
                  .First(si => si.Name == "ArcGIS 2D");

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  NorthArrowStyleItem naStyleItem = arcgis2dStyles.SearchNorthArrows(
                "ArcGIS North 13").FirstOrDefault();

  //Reference the map frame and define the location
  MapFrame newFrame = layout.FindElement("New Map Frame") as MapFrame;
  Coordinate2D nArrow = new Coordinate2D(6, 2.5);

  //Construct the north arrow
  //At 2.x - var newNorthArrow = LayoutElementFactory.Instance.CreateNorthArrow(
  //                        layout, nArrow, newFrame, naStyleItem);

  var naInfo = new NorthArrowInfo()
  {
    MapFrameName = newFrame.Name,
    NorthArrowStyleItem = naStyleItem
  };
  var newNorthArrow = ElementFactory.Instance.CreateMapSurroundElement(
                          layout, nArrow.ToMapPoint(), naInfo);
});

//Must be on QueuedTask.Run(() => { ...

//Build geometry
Coordinate2D ll = new Coordinate2D(5.0, 6);
Coordinate2D ur = new Coordinate2D(6.0, 7);
Envelope sbEnv = EnvelopeBuilderEx.CreateEnvelope(ll, ur);

//Reference a Scale Bar in a style
StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>()
              .FirstOrDefault(item => item.Name == "ArcGIS 2D");
ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars(
             "Alternating Scale Bar 1")[0];
//ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars(
//                                   "Double Alternating Scale Bar 1")[0];
//ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars(
//                                    "Hollow Scale Bar 1")[0];

//Create Scale Bar
ScaleBarInfo sbInfo = new ScaleBarInfo()
{
  MapFrameName = mapFrame.Name
};

var sbElm = ElementFactory.Instance.CreateMapSurroundElement(
                                   layout, sbEnv, sbInfo) as ScaleBar;

//Must be on QueuedTask.Run(() => { ...

//Build geometry
Coordinate2D ll = new Coordinate2D(5.0, 8);
Coordinate2D ur = new Coordinate2D(6.0, 9);
Envelope sbEnv = EnvelopeBuilderEx.CreateEnvelope(ll, ur);

//Reference a Scale Bar in a style
StyleProjectItem stylePrjItm = Project.Current.GetItems<StyleProjectItem>()
                             .FirstOrDefault(item => item.Name == "ArcGIS 2D");
ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars(
                              "Scale Line 1")[0];
//ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars(
//                                           "Stepped Scale Line")[0];
//ScaleBarStyleItem sbStyleItm = stylePrjItm.SearchScaleBars(
//                                            "Scale Line 2")[0];

//Create Scale Bar
ScaleBarInfo sbInfo = new ScaleBarInfo()
{
  MapFrameName = mapFrame.Name,
  ScaleBarStyleItem = sbStyleItm
};

var sbElm = ElementFactory.Instance.CreateMapSurroundElement(
                  layout, sbEnv, sbInfo, "ScaleBar Line") as ScaleBar;

//Get all styles in the project
var ProjectStyles = Project.Current.GetItems<StyleProjectItem>();

//Get a specific style in the project by name
StyleProjectItem style = ProjectStyles.First(x => x.Name == "NameOfTheStyle");