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开发环境：C#，VS2019，.NET Core 3.1，ASP.NET Core API

1、建立一个验证码控制器

新建两个方法Create和Check，Create用于创建验证码，Check用于验证它是否有效。

声明一个静态类变量存放列表，列表中存放包含令牌和验证码的对象。

        private static List<VCodeCalculateModel> _list = new List<VCodeCalculateModel>();/// <summary>/// 返回一个图形验证码和令牌./// </summary>/// <returns></returns>[HttpGet]public string Create(){VCodeCalculateModel model = new VCodeCalculateModel();model.id = Guid.NewGuid().ToString();    // 生成令牌var codes = VCodeCalculateModel.GetVCode();var num1 = codes.Item1;var num2 = codes.Item2;var bool_operator = codes.Item3;if (bool_operator){// 加法运算，生成验证码model.code = Convert.ToString(num1 + num2);}else{// 减法运算，生成验证码model.code = Convert.ToString(num1 - num2);}_list.Add(model);// 返回对象VCodeCalculateController_Create_Receive result = new VCodeCalculateController_Create_Receive();result.code = "0";result.data = new VCodeCalculateController_Create_Receive.Data();result.data.id = model.id;var image = VCodeCalculateModel.DrawImage(num1, num2, bool_operator);var base64 = Utils.BitmapToBase64Str(image);result.data.img = base64;var json = JsonConvert.SerializeObject(result);return json;}/// <summary>/// 检查验证码是否有效/// </summary>/// <param name="id">令牌.</param>/// <param name="code">验证码.</param>/// <returns></returns>[HttpGet]public string Check(string id, string code){try{var index = _list.FindIndex(m =>{if (m.id.Equals(id) && m.code.Equals(code)){return true;}return false;});ReceiveObject result = new ReceiveObject();if (index >= 0){_list.RemoveAt(index);result.code = "0";result.msg = "验证成功";var json = JsonConvert.SerializeObject(result);return json;}else{result.code = "1";result.msg = "验证失败";var json = JsonConvert.SerializeObject(result);return json;}}catch (Exception ex){_logger.LogError(exception: ex, message: ex.Message);ReceiveObject result = new ReceiveObject();result.code = "999999";result.msg = "系统异常";var json = JsonConvert.SerializeObject(result);return json;}}

2、建立一个验证码模型

验证码模型类包括：令牌和验证码属性。

再创建一个类存放Create方法返回的对象包括：令牌，验证码图片。

创建用于生成2个随机数和运算法的方法。

创建将2个随机数和运算法绘制成图片的方法。

        /// <summary>/// 令牌./// </summary>public string id { get; set; }/// <summary>/// 验证码./// </summary>public string code { get; set; }/// <summary>/// 计算数字列表./// </summary>static readonly int[] list = new int[50]{1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 20,21, 22, 23, 24, 25, 26, 27, 28, 29, 30,31, 32, 33, 34, 35, 36, 37, 38, 39, 40,41, 42, 43, 44, 45, 46, 47, 48, 49, 50,};/// <summary>/// 获取随机验证码./// </summary>/// <returns>item1 - 第一个数字，item2 - 第二个数字，item3 - 运算符（true - 加法， false - 减法）</returns>public static Tuple<int, int, bool> GetVCode(){Random random = new Random();var index = random.Next(0, list.Length);int num1 = list[index];var index2 = random.Next(0, list.Length);int num2 = list[index2];bool bool_operator = random.Next(0, 2) == 1 ? true : false;return new Tuple<int, int, bool>(num1, num2, bool_operator);}/// <summary>/// 绘制验证码的图片./// </summary>/// <param name="code"></param>/// <returns></returns>public static Bitmap DrawImage(int num1, int num2, bool isAdd){Color[] list_color ={Color.FromArgb(240, 230, 140),    // 黄褐色（亮）Color.FromArgb(138, 54, 15),    // 黄褐色（暗）Color.FromArgb(51, 161, 201),    // 蓝色（亮）Color.FromArgb(25, 25, 112),    // 蓝色（暗）Color.FromArgb(192, 192, 192),    // 灰白（亮）Color.FromArgb(128, 128, 105),    // 灰白（暗）};Random random = new Random();// 创建画板Bitmap bitmap = new Bitmap(150, 50);// 创建画笔Graphics grp = Graphics.FromImage(bitmap);grp.Clear(Color.White);    // 设置背景色为白色// 绘制噪点for (int i = 0; i < random.Next(60, 80); i++){int x = random.Next(bitmap.Width);int y = random.Next(bitmap.Height);grp.DrawLine(new Pen(Color.LightGray, 1), x, y, x + 1, y);}// 绘制随机的线条grp.DrawLine(new Pen(list_color[random.Next(list_color.Length)], random.Next(3)),new Point(random.Next(bitmap.Width / 2), random.Next(bitmap.Height / 2)),new Point(bitmap.Width / 2 + random.Next(bitmap.Width / 2), bitmap.Height / 2 + random.Next(bitmap.Height / 2)));// 绘制验证码string code = string.Format("{0}{1}{2}", num1, isAdd ? "+" : "-", num2);for (int i = 0; i < code.Length; i++){var item = code[i];grp.DrawString(item.ToString(),new Font(FontFamily.GenericSansSerif, 25, FontStyle.Bold),new SolidBrush(list_color[random.Next(list_color.Length)]),x: (150 / 5) * i,y: random.Next(5));}// 在验证码上绘制噪点for (int i = 0; i < random.Next(30, 50); i++){int x = random.Next(bitmap.Width);int y = random.Next(bitmap.Height);grp.DrawLine(new Pen(list_color[random.Next(list_color.Length)], 1), x, y, x + 1, y);}// 绘制随机的线条grp.DrawLine(new Pen(list_color[random.Next(list_color.Length)], random.Next(3)),new Point(random.Next(bitmap.Width / 2), random.Next(bitmap.Height / 2)),new Point(bitmap.Width / 2 + random.Next(bitmap.Width / 2), bitmap.Height / 2 + random.Next(bitmap.Height / 2)));return bitmap;}/// <summary>/// VCodeCalculateController控制器中Create方法的返回对象./// </summary>public class VCodeCalculateController_Create_Receive : ReceiveObject{public Data data { get; set; }public class Data{/// <summary>/// 令牌./// </summary>public string id { get; set; }/// <summary>/// Base64的验证码图片./// </summary>public string img { get; set; }}}

运行效果：