1. 劃線乘法

namespace Q1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        int? a, b;

        private void button1_Click(object sender, EventArgs e)
        {
            a = int.Parse(textBox1.Text);
            b = int.Parse(textBox2.Text);
            panel1.Refresh();
        }

        private void panel1_Paint(object sender, PaintEventArgs e)
        {
            if (a == null) return;
            {
                var r = a.Value / 10;
                var pen = new Pen(Brushes.Red, 2);
                if (r == 0) pen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dash;
                for (int i = 0; i < Math.Max(r, 1); i++)
                {
                    e.Graphics.DrawLine(
                pen,
                  50 + 200 + i * 10 + 50,50 + 0 + i * 10 - 50,50 + 0 + i * 10 + 50,50 + 200 + i * 10 - 50
                        );
                }

                var k = a.Value % 10;
                pen.DashStyle = k == 0 ? System.Drawing.Drawing2D.DashStyle.Dash : System.Drawing.Drawing2D.DashStyle.Solid;
                for (int i = 0; i < Math.Max(1, k); i++)
                {
                    e.Graphics.DrawLine(
                     pen,
                     50 + 400 + i * 10,50 +50 + i * 10,50 + 150 + i * 10,50 + 300 + i * 10
                        );
                }
            }

            {
                var r = b.Value / 10;
                var pen = new Pen(Brushes.Blue, 2);
                if (r == 0) pen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dash;
                var z = +50;
                for (int i = 0; i < Math.Max(r, 1); i++)
                {
                    e.Graphics.DrawLine(
                pen,
                     50 + i * -10 - 75 + z,50 + i * 10,50 + 300 + z + i * -10 - 75 + z,50 + 300 + i * 10 + z
                        );
                }

                var k = b.Value % 10;
                pen.DashStyle = k == 0 ? System.Drawing.Drawing2D.DashStyle.Dash : System.Drawing.Drawing2D.DashStyle.Solid;
                for (int i = 0; i < Math.Max(1, k); i++)
                {
                    e.Graphics.DrawLine(
                     pen,
                      50 + 200 + i * -10 - 75 + z,50 + i * 10 - 75 + z,50 + 500 + i * -10 - 75 + z,50 + 300 + i * 10 - 75 + z
                        );
                }
            }
        }
    }
}