（１）加減乗除の計算
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Sample2
{
    class CSHAPRO0301
    {
        static void Main(string[] args)
        {
            Console.Write("x = ");
            double x = double.Parse(Console.ReadLine());
            Console.Write("y = ");
            double y = double.Parse(Console.ReadLine());
            Console.Write("x + y = {0}\n", x + y);
            Console.Write("x - y = {0}\n", x - y);
            Console.Write("x * y = {0}\n", x * y);
            Console.Write("x / y = {0}\n", x / y);
            Console.Write("x % y = {0}\n", x % y);
            Console.ReadKey();
        }
    }
}


（２）クリックで円を描画
using System;
using System.Windows.Forms;
using System.Drawing;
using System.Collections.Generic;

class Sample : Form
{
    private List<Point> ls;

    public static void Main()
    {
        Application.Run(new Sample());
    }
    public Sample()
    {
        this.Text = "サンプル";
        this.Width = 600; this.Height = 600;

        ls = new List<Point>();

        this.MouseDown += new MouseEventHandler(fm_Click);
        this.Paint += new PaintEventHandler(fm_Paint);
    }

    public void fm_Click(object sender, MouseEventArgs e)
    {
        Point p = new Point();
        p.X = e.X;
        p.Y = e.Y;
        ls.Add(p);
        this.Invalidate();
    }
    public void fm_Paint(object sender, PaintEventArgs e)
    {
        Graphics g = e.Graphics;
        Pen dp = new Pen(Color.Black, 1);

        foreach ( Point p in ls )
        {
            int x = p.X;
            int y = p.Y;
            g.DrawEllipse(dp, x, y, 10, 10);
        }
    }
}