DropDownButton.cs

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

namespace SleepyBird.View.Controls
{

    /// <summary>
    /// DropDownButton
    /// Reference: http://akabeko.sakura.ne.jp/blog/2009/10/wpf-%E3%81%A7-dropdown-%E3%83%A1%E3%83%8B%E3%83%A5%E3%83%BC%E3%83%9C%E3%82%BF%E3%83%B3/
    /// </summary>
    public class DropDownButton : ToggleButton
    {

        public DropDownButton()
        {
            var binding = new Binding("DropDownContextMenu.IsOpen") { Source = this };
            this.SetBinding(DropDownButton.IsCheckedProperty, binding);
        }

        public ContextMenu DropDownContextMenu
        {
            get
            {
                return this.GetValue(DropDownContextMenuProperty) as ContextMenu;
            }
            set
            {
                this.SetValue(DropDownContextMenuProperty, value);
            }
        }

        protected override void OnClick()
        {
            if (DropDownContextMenu == null) return;
            DropDownContextMenu.PlacementTarget = this;
            DropDownContextMenu.Placement = PlacementMode.Bottom;
            DropDownContextMenu.IsOpen = !DropDownContextMenu.IsOpen;
        }

        public static readonly DependencyProperty DropDownContextMenuProperty = DependencyProperty.Register("DropDownContextMenu", typeof(ContextMenu), typeof(DropDownButton), new UIPropertyMetadata(null));
    }
}