ASP.NET 純數字驗證碼產生器 (GDI+ 繪圖)

使用 System.Drawing 命名空間動態產生包含干擾線、隨機字體/顏色/位移的數字驗證碼圖片，儲存於 Session 供後續驗證。

驗證碼核心類別 (CheckCode.cs)

public class CheckCode
{
    // 公開靜態方法：產生驗證碼並輸出圖片
    public static void DrawImage()
    {
        CheckCode instance = new CheckCode();
        string code = instance.GenerateRandomCode(4); // 4 位數字
        HttpContext.Current.Session["CheckCode"] = code;
        instance.RenderImage(code);
    }

    // 產生不重複隨機數字
    private string GenerateRandomCode(int length)
    {
        string chars = "0,1,2,3,4,5,6,7,8,9";
        string[] arr = chars.Split(',');
        StringBuilder sb = new StringBuilder();
        Random rand = new Random();
        int last = -1;

        for (int i = 0; i < length; i++)
        {
            int idx;
            do { idx = rand.Next(arr.Length); } while (idx == last);
            last = idx;
            sb.Append(arr[idx]);
        }
        return sb.ToString();
    }

    // 繪製並輸出 JPEG
    private void RenderImage(string code)
    {
        int width = code.Length * 13;
        using (Bitmap bmp = new Bitmap(width, 23))
        using (Graphics g = Graphics.FromImage(bmp))
        {
            g.Clear(Color.White);

            // 干擾點
            Random rand = new Random();
            for (int i = 0; i < 50; i++)
            {
                g.DrawRectangle(new Pen(Color.LightGray), 
                    rand.Next(width), rand.Next(23), 1, 1);
            }

            // 字體與顏色陣列
            string[] fonts = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "新細明體" };
            Color[] colors = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };

            for (int i = 0; i < code.Length; i++)
            {
                int cIdx = rand.Next(colors.Length);
                int fIdx = rand.Next(fonts.Length);
                using (Font font = new Font(fonts[fIdx], 10, FontStyle.Bold))
                using (Brush brush = new SolidBrush(colors[cIdx]))
                {
                    int y = (i % 2 == 0) ? 4 : 2; // 錯位效果
                    g.DrawString(code[i].ToString(), font, brush, 3 + i * 12, y);
                }
            }

            // 邊框
            g.DrawRectangle(Pens.Black, 0, 0, width - 1, 22);

            // 輸出
            HttpContext.Current.Response.ClearContent();
            HttpContext.Current.Response.ContentType = "image/jpeg";
            using (MemoryStream ms = new MemoryStream())
            {
                bmp.Save(ms, ImageFormat.Jpeg);
                HttpContext.Current.Response.BinaryWrite(ms.ToArray());
            }
        }
    }
}

HTTP Handler 頁面 (CheckCode.aspx.cs)

public partial class CheckCodePage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        CheckCode.DrawImage();
    }
}

前端使用方式

<asp:TextBox ID="txtCode" runat="server" Width="150px" />
<img id="imgCaptcha" 
     src="CheckCode.aspx" 
     alt="看不清？點擊更換" 
     style="cursor:pointer; vertical-align:middle;" 
     onclick="this.src=this.src+'?'+Math.random()" />
<br />
<asp:Button ID="btnSubmit" runat="server" Text="送出" OnClick="btnSubmit_Click" />

後端驗證

protected void btnSubmit_Click(object sender, EventArgs e)
{
    string userInput = txtCode.Text.Trim();
    string sessionCode = Session["CheckCode"]?.ToString() ?? "";

    if (string.Equals(userInput, sessionCode, StringComparison.OrdinalIgnoreCase))
    {
        // 驗證通過
        lblMsg.Text = "驗證成功！";
    }
    else
    {
        // 驗證失敗
        ClientScript.RegisterStartupScript(this.GetType(), "alert", 
            "alert('驗證碼錯誤，請重新輸入！');", true);
    }
}

部署注意事項

• IIS 需啟用「應用程式集區身分識別」具備桌面互動權限，或設定 `Load User Profile = True`，否則 GDI+ 可能拋出「Generic GDI+ error」。
• 生產環境建議改用 System.Drawing.Common (跨平台) 或第三方庫 (如 SkiaSharp, ImageSharp) 避免 GDI+ 記憶體洩漏。
• 可擴充為英數混合、中文、算術題等多種驗證碼模式。