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>();
  // example: projectStyleName = "ArcGIS 3D";
  StyleProjectItem testStyle = container.FirstOrDefault(style => style.Name == projectStyleName);
  StyleItem cone = null;
  if (testStyle != null)
  {
    // example: symbolName = "Cone_Volume_3";
    cone = testStyle.LookupItem(StyleItemType.PointSymbol, symbolName);
  }
  // Use (testStyle, cone);
}

{
  // Note: Needs QueuedTask to run
  var containerStyle = Project.Current.GetProjectItemContainer("Style");
  var fav_style_item = 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);
}

{
  //Note: 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(
    layout, linePl, lineSym, "New Line");
}

{
  //Note: 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(
    layout, graphic, "New Point", true, elemInfo);
}

{
  //Create a simple 2D point graphic and apply an existing point style item as the symbology.
  //Note: Must be on 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);  

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

{
  //Create a simple 2D line graphic and apply an existing line
  //style item as the symbology.
  //Note: Must be on 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));

  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);  


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

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

  //Note: Must be on QueuedTask.Run

  //Build 2D envelope geometry
  Coordinate2D rec_ll = new Coordinate2D(1.0, 4.75);
  Coordinate2D rec_ur = new Coordinate2D(3.0, 5.75);

  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);


  GraphicElement recElm = ElementFactory.Instance.CreateGraphicElement(
    layout, rec_env, polySym, "New Rectangle");
  //Or use Predefined shape
  GraphicElement recElm2 = ElementFactory.Instance.CreatePredefinedShapeGraphicElement(
                            layout, 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");

  //Note: Must be on 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
  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

  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

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

{
  //Note: 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;
}

{
  //Note: 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 styleFound = ProjectStyles.First(x => x.Name == "NameOfTheStyle");
}