程序代码：

/*

 * Created by SharpDevelop.

 * User: hellson

 * Date: 2008-10-8

 * Time: 17:00

 * 

 * To change this template use Tools | Options | Coding | Edit Standard Headers.

 */

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace draws
{
    /// <summary>
    /// Description of MainForm.
    /// </summary>
    public partial class MainForm
    {
        Form f = new Form();
        int a=0,b=0;
        [STAThread]
        public static void Main(string[] args)
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MainForm());
        }
        
        public MainForm()
        {
            //
            // The InitializeComponent() call is required for Windows Forms designer support.
            //
            InitializeComponent();
            
            //
            // TODO: Add constructor code after the InitializeComponent() call.
            //
        }
        
        private void Button1Click(object sender, EventArgs e)
        {
            a = Convert.ToInt32(textBox1.Text);
            if (textBox2.Visible)
                b = Convert.ToInt32(textBox2.Text);
            this.Hide();

            f.Paint += new System.Windows.Forms.PaintEventHandler(this.MainFormPaint);
            f.Show();
        }
        
        void MainFormPaint(object sender, System.EventArgs e){
            Graphics g = f.CreateGraphics();
            Pen pen = new Pen(Color.Red, 2);
            switch (comboBox1.Text)
            {
                case "正方形":
                    {
                       
                        g.DrawRectangle(pen, 10, 10,a, a);
                        break;
                    }
                case "长方形":
                    {

                        g.DrawRectangle(pen, 10, 10, a, b);
                        break;
                    }
                case "圆":
                    {
                        g.DrawEllipse(pen, 10, 10, a, a);
                        break;
                    }

            }
            g.Dispose();
            f.Show();
        
        }
        
        void ComboBox1SelectedIndexChanged(object sender, System.EventArgs e)
        {
            switch (comboBox1.Text)
            {
                case "正方形":
                    { label1.Text = "正方形的边长";
                   
                        //a= Convert.ToInt32 (textBox1.Text);
                    label2.Visible = false;
                    textBox2.Visible = false;
                    label3.Visible = false;
                    textBox3.Visible = false;
                   
                        break;
                }
            case "长方形":
                {
                    label1.Text = "长方形的长";
                    //int a = Convert.ToInt32(textBox1.Text);
                    //int b = Convert.ToInt32(textBox2.Text);
                    label2.Text = "长方形的宽";
                    label2.Visible = true;
                    textBox2.Visible = true;

                    label3.Visible = false;
                    textBox3.Visible = false;
                    /*Graphics g = f.pictureBox1.CreateGraphics();
                    Pen pen = new Pen(Color.Red, 2);
                    g.DrawRectangle(pen, 10, 10, a, b);*/
                    break;
                }
                case"圆形":
                    {label1.Text = "圆形的半径";
                       
                    
                    label2.Visible = false;
                    textBox2.Visible = false;
                    label3.Visible = false;
                    textBox3.Visible = false;
                   
                        break;
                    }
            }
        }
    }
}