private int codeLen = 4;//驗證碼長度
private int fineness = 85;//圖片清晰度
private int imgWidth = 48;//圖片寬度
private int imgHeight = 24;//圖片高度
private string fontFamily = "Times New Roman";//字體名稱
private int fontSize = 14;//字體大小
private int fontStyle = 0;//字體樣式
private int posX = 0;//繪制起始坐標X
private int posY = 0;//繪制坐標Y
private string CreateValidateCode() //生成驗證碼
{
string validateCode = "";
Random random = new Random();// 隨機數對象
for (int i = 0; i < codeLen; i++)//循環生成每位數值
{
int n = random.Next(10);//數字
validateCode += n.ToString();
}
Session["vcode"] = validateCode;//保存驗證碼
return validateCode;// 返回驗證碼
}
private void DisturbBitmap(Bitmap bitmap)//圖像背景
{
Random random = new Random();//通過隨機數生成
for (int i = 0; i < bitmap.Width; i++)//通過循環嵌套，逐個像素點生成
{
for (int j = 0; j < bitmap.Height; j++)
{
if (random.Next(90) <= this.fineness)
bitmap.SetPixel(i,j,Color.LightGray);
}
}
}
private void DrewValidateCode(Bitmap bitmap,string validateCode)//繪制驗證碼圖像
{
Graphics g = Graphics.FromImage(bitmap);//獲取繪制器對象
Font font = new Font(fontFamily,fontSize,FontStyle.Bold);//設置繪制字體
g.DrawString(validateCode,font,Brushes.Black,posX,posY);//繪制驗證碼圖像
}

protected void Page_Load(object sender, EventArgs e)
{
string validateCode = CreateValidateCode();//生成驗證碼
Bitmap bitmap = new Bitmap(imgWidth,imgHeight);//生成Bitmap圖像
DisturbBitmap(bitmap); //圖像背景
DrewValidateCode(bitmap,validateCode);//繪制驗證碼圖像
bitmap.Save(Response.OutputStream,ImageFormat.Gif);//保存圖像，等待輸出
}

<asp:Image ID="Image1" runat="server" Height="21px" Width="61px" ImageUrl="~/Default2.aspx" ImageAlign="Middle" />