1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:

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;
            }
        }
    }					

1:
2:
3:
4:
5:
6:

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

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

1:

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

1:

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

1:

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

1:

var myNewPoint = Point1 + Point2 + Point3 - Point4;					

1:

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