Code
using System;
using System.Collections.Generic;
using System.Web;
using System.Security.Cryptography;
using System.Drawing;
using System.IO;
using System.Drawing.Imaging;

namespace Test.CheckCode
{
    /// <summary>
    /// 举行左上角坐标和X、Y距离
    /// </summary>
    public class Coordinate
    {
        private int x;
        private int y;
        private int xDistance = 0;
        private int yDistance = 0;

/// <summary>
        /// X距离
        /// </summary>
        public int XDistance
        {
            get { return xDistance; }
            set { xDistance = value; }
        }

/// <summary>
        /// Y距离
        /// </summary>
        public int YDistance
        {
            get { return yDistance; }
            set { yDistance = value; }
        }

/// <summary>
        /// X点坐标
        /// </summary>
        public int X
        {
            get { return x; }
            set { x = value; }
        }

/// <summary>
        /// Y点坐标
        /// </summary>
        public int Y
        {
            get { return y; }
            set { y = value; }
        }
    }

/// <summary>
    /// 反色矩形集合生产器
    /// </summary>
    public class ReverseCoordinates
    {
        /// <summary>
        /// 单件模式 实例
        /// </summary>
        public static readonly ReverseCoordinates Instance = new ReverseCoordinates();
        /// <summary>
        /// 总长度
        /// </summary>
        public const int TotalLenght = 650;

/// <summary>
        /// 总宽度
        /// </summary>
        public const int TotalHeight = 250;
        private List<Coordinate> list = new List<Coordinate>();

private ReverseCoordinates()
        { }

/// <summary>
        /// 返回要反色矩形左上角坐标的集合
        /// </summary>
        /// <returns>反色矩形左上角坐标的集合</returns>
        public List<Coordinate> GetList()
        {
            this.CreateList();

return this.list;
        }

/// <summary>
        /// 创建反色矩形左上角坐标的集合
        /// </summary>
        private void CreateList()
        {
            Random random = new Random(Guid.NewGuid().GetHashCode());

//创建4个角反色矩形的长和宽
            int cornerXDistance = random.Next(1, Convert.ToInt32(TotalLenght / 15));
            int cornerYDistance = random.Next(1, Convert.ToInt32(TotalLenght / 5));

this.list = new List<Coordinate>();

//创建左上角反色矩形
            Coordinate coordinate = new Coordinate();

coordinate.X = 1;
            coordinate.Y = 1;
            coordinate.XDistance = cornerXDistance;
            coordinate.YDistance = cornerYDistance;

this.list.Add(coordinate);

//创建左下角反色矩形
            coordinate = new Coordinate();

coordinate.X = 1;
            coordinate.Y = TotalHeight - cornerYDistance;
            coordinate.XDistance = cornerXDistance;
            coordinate.YDistance = cornerYDistance;

this.list.Add(coordinate);

//创建右上角反色矩形
            coordinate = new Coordinate();

coordinate.X = TotalLenght - cornerXDistance;
            coordinate.Y = 1;
            coordinate.XDistance = cornerXDistance;
            coordinate.YDistance = cornerYDistance;

this.list.Add(coordinate);

//创建右下角反色矩形
            coordinate = new Coordinate();

coordinate.X = TotalLenght - cornerXDistance;
            coordinate.Y = TotalHeight - cornerYDistance;
            coordinate.XDistance = cornerXDistance;
            coordinate.YDistance = cornerYDistance;

this.list.Add(coordinate);

//创建中间大的反色矩形
            coordinate = new Coordinate();

//得到每个字的大概大小
            int fontSize = Convert.ToInt32(TotalLenght / 3.3);

//得到每个字大概中间的部位的X值
            int[] sizeArray = new int[4];

sizeArray[0] = fontSize;
            sizeArray[1] = fontSize * 1 - fontSize;
            sizeArray[2] = fontSize * 2 - fontSize;
            sizeArray[3] = fontSize * 3 - fontSize;

//随机选取中间反色矩形的X值
            int index = random.Next(0, 3);

//得到中间反色矩形和左上角矩形的距离
            int beginX = sizeArray[index] - cornerXDistance;

//如果两个矩形重叠
            if (beginX < 0)
            {
                //将负的距离变正数
                beginX *= -1;
            }
            else
            {
                beginX = 0;
            }

//为了每次中间的反色矩形位置都不一样计算偏移值
            int offset = random.Next(beginX, Convert.ToInt32(fontSize / 2));

coordinate.X = sizeArray[index] + offset;
            coordinate.Y = random.Next(1, Convert.ToInt32(TotalHeight / 2));
            coordinate.XDistance = random.Next(Convert.ToInt32(TotalLenght / 6), Convert.ToInt32(TotalLenght / 3));
            coordinate.YDistance = random.Next(Convert.ToInt32(TotalHeight / 2), TotalHeight - 1);

this.list.Add(coordinate);
        }

}

/// <summary>
    /// 验证码生成器
    /// </summary>
    public class ValidateCode
    {
        private string checkCode;

/// <summary>
        /// 验证码
        /// </summary>
        public string CheckCode
        {
            get { return checkCode; }
            set { checkCode = value; }
        }

public ValidateCode()
        {
            this.GenerateCheckCode();
        }

/// <summary>
        /// 创建随机字符串
        /// </summary>
        private void GenerateCheckCode()
        {
            int number;
            char code;
            this.checkCode = String.Empty;
            Random random = new Random();
            for (int i = 0; i < 4; i++)
            {
                number = random.Next();
                code = (char)('0' + (char)(number % 10));
                this.checkCode += code.ToString();
            }
        }

/// <summary>
        /// 生产图片
        /// </summary>
        public void CreateCheckCodeImage()
        {
            ReverseCoordinates coordinates = ReverseCoordinates.Instance;

int totalLenght = ReverseCoordinates.TotalLenght;
            int totalHeight = ReverseCoordinates.TotalHeight;

Bitmap image = null;
            Graphics graphics = null;
            MemoryStream ms = null;

try
            {
                image = new Bitmap(totalLenght + 1, totalHeight + 1);
                graphics = Graphics.FromImage(image);
                ms = new MemoryStream();

//清空图片背景色
                graphics.Clear(Color.White);
                Font fontA = new Font("Arial", Convert.ToInt32(totalLenght / 3.3), (FontStyle.Bold | FontStyle.Italic));

Random random = new Random(Guid.NewGuid().GetHashCode());
                int offset = random.Next(-5, 0);

graphics.DrawString(checkCode, fontA, Brushes.Black, offset, offset);
                
                Color pixel;

List<Coordinate> list = coordinates.GetList();

foreach (Coordinate coordinate in list)
                {
                    int x = coordinate.X;

for (int i = 0; i < coordinate.XDistance && x < totalLenght; i++, x++)
                    {
                        int y = coordinate.Y;

for (int j = 0; j < coordinate.YDistance && y < totalHeight; j++, y++)
                        {
                            int r, g, b;
                            pixel = image.GetPixel(x, y);

r = 255 - pixel.R;
                            g = 255 - pixel.G;
                            b = 255 - pixel.B;

image.SetPixel(x, y, Color.FromArgb(r, g, b));
                        }
                    }
                }

//画图片的边框线
                graphics.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

image.Save(ms, ImageFormat.Gif);

HttpResponse httpResponse = HttpContext.Current.Response;
                httpResponse.ClearContent();
                httpResponse.Expires = 0;
                httpResponse.ExpiresAbsolute = DateTime.Now.AddSeconds(-1);
                httpResponse.ContentType = "image/Gif";
                httpResponse.BinaryWrite(ms.ToArray());
            }
            finally
            {
                graphics.Dispose();
                image.Dispose();
                ms.Dispose();
            }

}
    }
}