DropdownButton.cs

using System.Windows.Controls.Primitives;
using System.Windows.Data;
using System.Windows.Media;
using System.Windows.Shapes;

//https://github.com/givemelight/wpf-toolbar-dropdown

namespace System.Windows.Controls.Extensions {
    public class DropdownButton : ToggleButton {
        public DropdownButton() {
            Checked += DropdownButton_Checked;
            ContentTemplate = GetDataTemplate();
            Style = (Style)FindResource(ToolBar.ToggleButtonStyleKey);
        }

        private DataTemplate GetDataTemplate() {
            DataTemplate template = new DataTemplate(typeof(DropdownButton));
            FrameworkElementFactory path = new FrameworkElementFactory(typeof(Path));
            path.SetValue(Path.DataProperty, Geometry.Parse("F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z"));
            path.SetValue(MarginProperty, new Thickness(4, 4, 0, 4));
            path.SetValue(WidthProperty, 5d);
            path.SetValue(Shape.FillProperty, Brushes.Black);
            path.SetValue(Shape.StretchProperty, Stretch.Uniform);
            path.SetValue(HorizontalAlignmentProperty, HorizontalAlignment.Right);
            path.SetValue(VerticalAlignmentProperty, VerticalAlignment.Center);

            FrameworkElementFactory panel = new FrameworkElementFactory(typeof(StackPanel));
            panel.SetValue(StackPanel.OrientationProperty, Orientation.Horizontal);

            FrameworkElementFactory contentPresenter = new FrameworkElementFactory(typeof(ContentPresenter));

            Binding binding = new Binding("Content") { Source = this };
            contentPresenter.SetBinding(ContentProperty, binding);

            panel.AppendChild(contentPresenter);
            panel.AppendChild(path);

            template.VisualTree = panel;
            return template;
        }

        private void DropdownMenu_Closed(object sender, System.Windows.RoutedEventArgs e) {
            IsChecked = false;
        }

        private ContextMenu dropdownMenu;
        public ContextMenu DropdownMenu {
            get {
                return dropdownMenu;
            }
            set {
                dropdownMenu = value;
                dropdownMenu.Closed += DropdownMenu_Closed;
            }
        }

        private void DropdownButton_Checked(object sender, System.Windows.RoutedEventArgs e) {
            DropdownMenu.PlacementTarget = sender as ToggleButton;
            DropdownMenu.Placement = PlacementMode.Bottom;
            DropdownMenu.IsOpen = true;
        }
    }
}