using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace demo10
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        
        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;
            Rectangle rect = new Rectangle();
            //圆的直径等于绘图区域的最短边
            float diameter = Math.Min(rect.Width, rect.Height);
            //半径
            float radius = diameter / 2;
            //圆心
            PointF center = new PointF(rect.X + rect.Width / 2, rect.Height / 2);
            //画几个圆，先试试5个
            int count = 5;
            float diameterStep = diameter / count;
            float radiusStep = radius / count;
            //生成圆的范围
            RectangleF cirleRect = new RectangleF();
            cirleRect.X = center.X - radius;
            cirleRect.Y = center.Y - radius;
            cirleRect.Width = cirleRect.Height = diameter;
            //画同心圆
            for (int i = 0; i < count; i++)
            {
                g.DrawEllipse(Pens.Gray, cirleRect);
                cirleRect.X += radiusStep;
                cirleRect.Y += radiusStep;
                cirleRect.Width -= diameterStep;
                cirleRect.Height -= diameterStep;
            }

        }

    }
}

你的 rect 里面是空的，应该改成form的长和宽，或者对rect 进行赋值。
对rect进行赋值加上一句：   
   rect = this.RestoreBounds;





[ 本帖最后由 over1230 于 2015-1-26 09:25 编辑 ]