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public partial class MS_SplitButton : UserControl
    {
        private string[] _items;
        ContextMenuStrip strip = new ContextMenuStrip();

        public MS_SplitButton()
        {
            InitializeComponent();
        }

        private void pictureBox2_Click(object sender, EventArgs e)
        {
            foreach (string it in _items)
                strip.Items.Add(it);
            strip.Show(this, this.Location.Y, this.Location.X);
            strip.Items.Clear();
        }

        [DisplayName("ContextMenu Items")]
        public string[] Items
        {
            get
            {
                return _items;
            }
            set
            {
                _items = value;
            }
        }
    }                    

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Point splitButtonLocation = ...
Size splitButtonSize = ...

var verticalMargin = 5;
var menuLocation = splitButtonLocation + new Point(0, splitButtonSize.Height + verticalMargin);
var menuSize = new Size(splitButtonSize.Width, CalculateMenuHeight());                    

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var menuLocation = new Point(splitButtonLocation.X, splitButtonLocation.Y + splitButtonSize.Height + verticalMargin);                    

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strip.Show(this, new Point(0, Height), ToolStripDropDownDirection.BelowRight);                    

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strip.Show(this, new Point(Width, Height), ToolStripDropDownDirection.BelowLeft);                    

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var myNewPoint = Point1 + Point2 + Point3 - Point4;                    

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var myNewPoint = new Point(Point1.X + Point2.X + Point3.X - Point4.X, Point1.Y + Point2.Y + Point3.Y - Point4.Y);