// 随机生成指定长度的验证码字符串
private string RandomCode(int length)
{
    string s = "0123456789zxcvbnmasdfghjklqwertyuiop";
    StringBuilder sb = new StringBuilder();
    Random rand = new Random();
    int index;
    for(int i = 0; i < length; i++)
    {
        index = rand.Next(0, s.Length);
        sb.Append(s[index]);
    }
    return sb.ToString();
}

private void PaintInterLine(Graphics g,int num,int width,int height)
{
    Random r = new Random();
    int startX, startY, endX, endY;
    for(int i = 0; i < num; i++)
    {
        startX = r.Next(0, width);
        startY = r.Next(0, height);
        endX = r.Next(0, width);
        endY = r.Next(0, height);
        g.DrawLine(new Pen(Brushes.Red), startX, startY, endX, endY);
    }
}

public ActionResult GetValidateCode()
{
    byte[] data = null;
    string code = RandomCode(5);
    TempData["code"] = code;
    //定义一个画板
    MemoryStream ms = new MemoryStream();
    using(Bitmap map=new Bitmap(100, 40))
    {
        //画笔,在指定画板画板上画图
        //g.Dispose();
        using (Graphics g = Graphics.FromImage(map))
        {
            g.Clear(Color.White);
            g.DrawString(code,new Font("黑体",18.0F),Brushes.Blue,new Point(10,8));
            //绘制干扰线
            PaintInterLine(g, 30, map.Width, map.Height);
        }
        map.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
    }
    data = ms.GetBuffer();
    return File(data, "image/jpeg");
}

<form method="post" id="form1" action="/ValidateCode/login">
    <div class="code">
        <input type="text" name="code" />
        <img id="code" src="/ValidateCode/GetValidateCode/" />
        <a style="text-decoration:none; cursor:pointer" id="chCode">看不清?换一个</a>
    </div>
    <div >
        <input type="submit"  value="登录" />
    </div>
</form>

public ActionResult Login()
{
    string code = Request.Form["code"].ToString();
    if (string.IsNullOrEmpty(code))
    {
        return Content("验证输不能为空");
    }
    if (!code.Equals(TempData["code"]))
    {
        return Content("验证输不正确");
    }
    return Content("验证输入正确");
}