public partial class Form1 : Form
    {
        Graphics g;

        List<PointF> P = new List<PointF>();

        int index = -1;
        int s = 4;

        Pen penPoly = new Pen(Color.Gray, 1f);
        Pen penCurve = new Pen(Color.Black, 3f);

        Brush brushP = new SolidBrush(Color.Gray);

        float B0(float t)
        {
            return (1 - t) * (1 - t) * (1 - t);
        }
        float B1(float t)
        {
            return 3 * t * (1 - t) * (1 - t);
        }
        float B2(float t)
        {
            return 3 * t * t * (1 - t);
        }
        float B3(float t)
        {
            return t * t * t;
        }
        float Bx(float t)
        {
            return B0(t) * P[0].X + B1(t) * P[1].X + B2(t) * P[2].X + B3(t) * P[3].X;
        }
        float By(float t)
        {
            return B0(t) * P[0].Y + B1(t) * P[1].Y + B2(t) * P[2].Y + B3(t) * P[3].Y;
        }
        void DrawBezier()
        {
            float x0, y0, x1, y1;
            float t = 0f;
            float h = 1f / 250f;
            x0 = Bx(t);
            y0 = By(t);
            while(t < 1f)
            {
                t += h;
                x1 = Bx(t);
                y1 = By(t);
                g.DrawLine(penCurve, x0, y0, x1, y1);
                x0 = x1;
                y0 = y1;
            }
        }

        public Form1()
        {
            InitializeComponent();
        }

        private void canvas_Paint(object sender, PaintEventArgs e)
        {
            g = e.Graphics;
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

            for (int i = 0; i < P.Count - 1; i++)
                g.DrawLine(penPoly, P[i], P[i + 1]);

            if (P.Count == 4)
                DrawBezier();

            for (int i = 0; i < P.Count; i++)
                g.FillRectangle(brushP, P[i].X - s, P[i].Y - s, 2 * s, 2 * s);
        }
        private void canvas_MouseDown(object sender, MouseEventArgs e)
        {
            for (int i = 0; i < P.Count; i++)
            {
                if (Math.Abs(P[i].X - e.X) <= s &&
                    Math.Abs(P[i].Y - e.Y) <= s)
                    index = i;
            }

            if (index == -1 && P.Count < 4)
            {
                P.Add(e.Location);
                index = P.Count - 1;
                canvas.Refresh();
            }
        }
        private void canvas_MouseMove(object sender, MouseEventArgs e)
        {
            if (index != -1)
            {
                P[index] = e.Location;
                canvas.Refresh();
            }
        }
        private void canvas_MouseUp(object sender, MouseEventArgs e)
        {
            index = -1;
        }
    }