在開發(fā)中，有時候生成驗證碼的場景目前還是存在的，本篇演示不依賴第三方組件，生成隨機驗證碼圖片。

先添加驗證碼接口

public interface ICaptcha
{
    /// <summary>
    /// 生成隨機驗證碼
    /// </summary>
    /// <param name="codeLength"></param>
    /// <returns></returns>
    Task<string> GenerateRandomCaptchaAsync(int codeLength = 4);

    /// <summary>
    /// 生成驗證碼圖片
    /// </summary>
    /// <param name="captchaCode">驗證碼</param>
    /// <param name="width">寬為0將根據(jù)驗證碼長度自動匹配合適寬度</param>
    /// <param name="height">高</param>
    /// <returns></returns>
    Task<CaptchaResult> GenerateCaptchaImageAsync(string captchaCode, int width = 0, int height = 30);
}

驗證碼返回模型

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; }
}

接下來實現(xiàn)接口，主要是依賴微軟的System.Drawing.Common組件，注意命名空間的引用

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)
    {
        //驗證碼顏色集合
        Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };

        //驗證碼字體集合
        string[] fonts = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial" };

        //定義圖像的大小，生成圖像的實例
        var image = new Bitmap(width == 0 ? captchaCode.Length * 25 : width, height);

        var g = Graphics.FromImage(image);

        //背景設(shè)為白色
        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);
        }

        //驗證碼繪制在g中
        for (var i = 0; i < captchaCode.Length; i++)
        {
            //隨機顏色索引值
            var cindex = random.Next(c.Length);

            //隨機字體索引值
            var findex = random.Next(fonts.Length);

            //字體
            var f = new Font(fonts[findex], 15, FontStyle.Bold);

            //顏色  
            Brush b = new SolidBrush(c[cindex]);

            var ii = 4;
            if ((i + 1) % 2 == 0)
                ii = 2;

            //繪制一個驗證字符  
            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);
    }
}

在控制器中注入調(diào)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");
    }
}

實際使用的時候，code就是本次生成的驗證碼，可以將其保存在session中，進(jìn)行驗證，或者其它方式。