asp.net core 图片验证码，后台验证

验证方法：

public static string VerificationCodeCacheFormat="vcode_cache_{0}";

public IActionResult ValidateCode()
        {
            VerificationCodeServices _vierificationCodeServices = new VerificationCodeServices();
            string code = "";
            System.IO.MemoryStream ms = _vierificationCodeServices.Create(out code);
            code = code.ToLower();//验证码不分大小写
            //HttpContext.Session.SetString("SupportValidateCode", code);
            Response.Body.Dispose();
            //ViewBag.code = code;
            var token = Guid.NewGuid().ToString();
            ViewBag.token = token;
            var cacheKey = string.Format(VerificationCodeCacheFormat, token);
            _memoryCache.Set(cacheKey, code, new MemoryCacheEntryOptions()
                    .SetSlidingExpiration(TimeSpan.FromMinutes()));
            CookieOptions options = new CookieOptions();
            Response.Cookies.Append("validatecode", token);
            return File(ms.ToArray(), @"image/png");
        }
        /// <summary>
        /// 验证码改为后台验证
        /// </summary>
        /// <param name="userToken"></param>
        /// <param name="userVerCode"></param>
        /// <returns></returns>
        public bool VerifyUserInputCode(string userToken, string userVerCode)
        {
            var cacheKey = string.Format(VerificationCodeCacheFormat, userToken.ToString());
            var vCode = "";
            if (!_memoryCache.TryGetValue(cacheKey, out vCode)) return false;
            if (vCode.ToLower() != userVerCode.ToLower()) return false;
            _memoryCache.Remove(cacheKey);
            return true;
        }

生成验证码及其图片

using System;
using System.Collections.Generic;
using System.DrawingCore;
using System.DrawingCore.Imaging;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
namespace homepage.Classes
{
    ///NuGet中引入第三方 ZKWeb.System.Drawing 3.0版本

    /// <summary>
    /// 图片验证码
    /// </summary>
    public class VerificationCodeServices
    {
        /// <summary>
        /// 生成指定长度的随机字符串
        /// </summary>
        /// <param name="codeLength">字符串的长度</param>
        /// <returns>返回随机数字符串</returns>
        private string RndomStr(int codeLength)
        {
            //组成字符串的字符集合  0-9数字、大小写字母
            string chars = "0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I,J,K,L,M,N,P,P,Q,R,S,T,U,V,W,X,Y,Z";

            string[] charArray = chars.Split(new Char[] { ',' });
            string code = "";
            int temp = -;//记录上次随机数值，尽量避避免生产几个一样的随机数
            Random rand = new Random();
            //采用一个简单的算法以保证生成随机数的不同
            for (int i = ; i < codeLength + ; i++)
            {
                if (temp != -)
                {
                    rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));//初始化随机类
                }
                int t = rand.Next();
                if (temp == t)
                {
                    return RndomStr(codeLength);//如果获取的随机数重复，则递归调用
                }
                temp = t;//把本次产生的随机数记录起来
                code += charArray[t];//随机数的位数加一
            }
            return code;
        }

        /// <summary>
        /// 将生成的字符串写入图像文件
        /// </summary>
        /// <param name="code">验证码字符串</param>
        /// <param name="length">生成位数（默认4位）</param>  

        public MemoryStream Create(out string code, int length = )
        {
            code = RndomStr(length);
            Bitmap Img = null;
            Graphics graphics = null;
            MemoryStream ms = null;
            Random random = new Random();
            //颜色集合
            Color[] color = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
            //字体集合
            string[] fonts = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
            //定义图像的大小，生成图像的实例
            Img = new Bitmap((int)code.Length * , );
            graphics = Graphics.FromImage(Img);//从Img对象生成新的Graphics对象
            graphics.Clear(Color.White);//背景设为白色  

            //在随机位置画背景点  

            for (int i = ; i < ; i++)
            {
                int x = random.Next(Img.Width);
                int y = random.Next(Img.Height);
                graphics.DrawRectangle(new Pen(Color.LightGray, ), x, y, , );
            }

            //验证码绘制在graphics中  

            for (int i = ; i < code.Length; i++)
            {
                int colorIndex = random.Next();//随机颜色索引值
                int fontIndex = random.Next();//随机字体索引值
                Font font = new Font(fonts[fontIndex], , FontStyle.Bold);//字体
                Brush brush = new SolidBrush(color[colorIndex]);//颜色
                int y = ;
                if ((i + ) %  == )//控制验证码不在同一高度
                {
                    y = ;
                }
                graphics.DrawString(code.Substring(i, ), font, brush,  + (i * ), y);//绘制一个验证字符
            }
            ms = new MemoryStream();//生成内存流对象
            Img.Save(ms, ImageFormat.Png);//将此图像以Png图像文件的格式保存到流中
            graphics.Dispose();
            Img.Dispose();
            return ms;
        }
    }
}

以上只适用于一个页面只有一个的验证码的情况，下面改进为可以一个页面多个验证码：

ValidateCode方法改为：

   public IActionResult ValidateCode(string identify = "")
        {
            VerificationCodeServices _vierificationCodeServices = new VerificationCodeServices();
            string code = "";
            System.IO.MemoryStream ms = _vierificationCodeServices.Create(out code);
            code = code.ToLower();//验证码不分大小写
            Response.Body.Dispose();
            var token = Guid.NewGuid().ToString();
            ViewBag.token = token;
            var cacheKey = string.Format(VerificationCodeCacheFormat, token);
            _memoryCache.Set(cacheKey, code, new MemoryCacheEntryOptions()
                    .SetSlidingExpiration(TimeSpan.FromMinutes()));
            CookieOptions options = new CookieOptions();
            Response.Cookies.Append("validatecode"+ identify, token);
            return File(ms.ToArray(), @"image/png");
        }

当页面只有一个验证码的时候，页面写法：

                                                      <p>
                                                            请输入验证码：<br><input type="text" id="validateCode" class="form-control" />
                                                            <img id="imgVerify" src="~/Home/ValidateCode" alt="看不清？点击更换" onclick="this.src = this.src + '?'" style="vertical-align:middle;" />
                                                        </p>

当页面多个验证码：

                                    <p>
                                            <input name="verification_code" type="text" maxlength="5" id="code_other" class="form-control tbText">
                                            <img id="imgVerifyOther" src="~/Home/ValidateCode?identify=other" alt="看不清？点击更换" onclick="this.src = 'ValidateCode?identify=other&' + Math.random()" style="vertical-align:middle;" />
                                        </p>