public partial class Form1 : Form
    {
        Point startpos, endpos;
        public Form1()
        {
            InitializeComponent();
        }
 
        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            startpos = new Point(e.X, e.Y);
        }
 
        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                endpos = new Point(e.X, e.Y);
                Invalidate();
            }
        }
        private void Figure(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;
 
            int width = endpos.X - startpos.X; //ここがわからないです
            int height = endpos.Y - startpos.Y; //
 
            SolidBrush brush = new SolidBrush(Color.Blue);
            g.FillEllipse(brush, startpos.X, startpos.Y, width, height);
        }
    }

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