先產生隨機亂數

並且把亂數存在session

/// <summary>
///  產生指定長度的隨機數
/// </summary>
/// <param name="length"></param>
/// <returns></returns>
private static 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();
}

HttpContext.Current.Session["CaptchaCode"] = code;

隨機顏色：

/// <summary>
/// 隨機Brushe
/// </summary>
/// <returns></returns>
private static Brush GetRandomBrushe() 
{
	Brush[] brus = new Brush[] { 
		Brushes.Black, 
		Brushes.Red, 
		Brushes.DarkBlue, 
		Brushes.Brown,
		Brushes.DarkCyan, 
		Brushes.Purple
	};
	Random rand = new Random();
    int index;
    index = rand.Next(0, brus.Length);
    return brus[index];
}

範例程式如下：

public static byte[] GetCaptchaCode(int len) 
{
	byte[] data;
	string code = RandomCode(len);
	HttpContext.Current.Session["CaptchaCode"] = code;
	MemoryStream ms = new MemoryStream();
	using (Bitmap map = new Bitmap(100, 40))
	using (Graphics g = Graphics.FromImage(map))
	{
		// 背景色
		g.Clear(Color.White);
		var brushe = GetRandomBrushe();
		g.DrawString(code, new Font("黑體", 18.0F), brushe, new Point(10, 8));
		//繪製干擾線(數字代表幾條)
		var lineColor = GetRandomBrushe();
		while (lineColor == brushe) 
		{
			lineColor = GetRandomBrushe();
		}
		PaintInterLine(g, lineColor, 10, map.Width, map.Height);
		// 繪製干擾點
		var pointColor = GetRandomBrushe();
		while (pointColor == lineColor || pointColor == brushe) 
		{
			pointColor = GetRandomBrushe();
		}
		PaintInterPoint(g, pointColor, 50, map.Width, map.Height);
		map.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
	}
	data = ms.GetBuffer();
	return data;
}

這邊是產生一個Byte之後產生回給前端一個檔案，但這個檔案並不存在伺服器上

不然驗證碼圖片的容量也蠻可觀的，驗證方式在拿Session跟輸入做核對就可以了

/// <summary>
///  取得及顯示驗證碼
/// </summary>
/// <returns></returns>
[HttpGet]
public ActionResult GetValidateCaptchaCode()
{
	byte[] data = CaptchaHelper.GetCaptchaCode(4);
	return File(data, "image/jpeg");
}

實作完成範例：

以下是驗證碼的搜尋結果：

C# 簡易驗證碼

2023/05/29

後端

驗證碼