在开发中，有时候生成验证码的场景目前还是存在的，本篇演示不依赖第三方组件，生成随机验证码图片。

先添加验证码接口

public interface ICaptcha
{/// <summary>/// 生成随机验证码/// </summary>/// <param name="codeLength"></param>/// <returns></returns>Task<string> GenerateRandomCaptchaAsync(int codeLength = 4);/// <summary>/// 生成验证码图片/// </summary>/// <param name="captchaCode">验证码</param>/// <param name="width">宽为0将根据验证码长度自动匹配合适宽度</param>/// <param name="height">高</param>/// <returns></returns>Task<CaptchaResult> GenerateCaptchaImageAsync(string captchaCode, int width = 0, int height = 30);
}

验证码返回模型

public class CaptchaResult
{/// <summary>/// CaptchaCode/// </summary>public string CaptchaCode { get; set; }/// <summary>/// CaptchaMemoryStream/// </summary>public MemoryStream CaptchaMemoryStream { get; set; }/// <summary>/// Timestamp/// </summary>public DateTime Timestamp { get; set; }
}

接下来实现接口，主要是依赖微软的System.Drawing.Common组件，注意命名空间的引用

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Threading.Tasks;...public class Captcha : ICaptcha
{private const string Letters = "1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z";public Task<CaptchaResult> GenerateCaptchaImageAsync(string captchaCode, int width = 0, int height = 30){//验证码颜色集合Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };//验证码字体集合string[] fonts = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial" };//定义图像的大小，生成图像的实例var image = new Bitmap(width == 0 ? captchaCode.Length * 25 : width, height);var g = Graphics.FromImage(image);//背景设为白色g.Clear(Color.White);var random = new Random();for (var i = 0; i < 100; i++){var x = random.Next(image.Width);var y = random.Next(image.Height);g.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);}//验证码绘制在g中for (var i = 0; i < captchaCode.Length; i++){//随机颜色索引值var cindex = random.Next(c.Length);//随机字体索引值var findex = random.Next(fonts.Length);//字体var f = new Font(fonts[findex], 15, FontStyle.Bold);//颜色  Brush b = new SolidBrush(c[cindex]);var ii = 4;if ((i + 1) % 2 == 0)ii = 2;//绘制一个验证字符  g.DrawString(captchaCode.Substring(i, 1), f, b, 17 + (i * 17), ii);}var ms = new MemoryStream();image.Save(ms, ImageFormat.Png);g.Dispose();image.Dispose();return Task.FromResult(new CaptchaResult{CaptchaCode = captchaCode,CaptchaMemoryStream = ms,Timestamp = DateTime.Now});}public Task<string> GenerateRandomCaptchaAsync(int codeLength = 4){var array = Letters.Split(new[] { ',' });var random = new Random();var temp = -1;var captcheCode = string.Empty;for (int i = 0; i < codeLength; i++){if (temp != -1)random = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));var index = random.Next(array.Length);if (temp != -1 && temp == index)return GenerateRandomCaptchaAsync(codeLength);temp = index;captcheCode += array[index];}return Task.FromResult(captcheCode);}
}

在控制器中注入调用

using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;[Route("api/[controller]")]
[ApiController]
public class CaptchaController : ControllerBase
{[HttpGet]public async Task<FileContentResult> CaptchaAsync([FromServices] ICaptcha _captcha){var code = await _captcha.GenerateRandomCaptchaAsync();var result = await _captcha.GenerateCaptchaImageAsync(code);return File(result.CaptchaMemoryStream.ToArray(), "image/png");}
}

实际使用的时候，code就是本次生成的验证码，可以将其保存在session中，进行验证，或者其它方式。