Create Captcha using C#

Step 1: Create aspx page GenerateCaptcha.aspx.

Step 2: In the code behind write the below code; 

Response.Clear();
int height = 30;
int width = 100;
Bitmap bmp = new Bitmap(width, height);
RectangleF rectf = new RectangleF(20, 5, 0, 0);
Graphics g = Graphics.FromImage(bmp);
g.Clear(Color.White);
g.SmoothingMode = SmoothingMode.AntiAlias;
g.InterpolationMode = InterpolationMode.HighQualityBicubic;
g.PixelOffsetMode = PixelOffsetMode.HighQuality;
g.DrawString(Session["captcha"].ToString(), new Font("Thaoma", 15, FontStyle.Strikeout), Brushes.Green, rectf);
g.DrawRectangle(new Pen(Brushes.LemonChiffon), 1, 1, width - 8, height - 2);
g.Flush();
Response.ContentType = "image/jpeg";
bmp.Save(Response.OutputStream, ImageFormat.Jpeg);
g.Dispose();
bmp.Dispose();

Step 3: Where you want to show captcha copy paste the below code,

<asp:UpdatePanel ID="UP1" runat="server">
    <ContentTemplate>
        <asp:Image ID="imgCaptcha" runat="server" />
        <asp:LinkButton ID="lbtnRefresh" CssClass="btn btn-info" runat="server" OnClick="btnRefresh_Click"><span class="glyphicon glyphicon-refresh"></span> Refresh</asp:LinkButton>
    </ContentTemplate>
</asp:UpdatePanel>

Step 4: In code behind make the function below and call on page load,

void FillCapctha()
{
    try
    {
        Random random = new Random();
        string combination = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
        StringBuilder captcha = new StringBuilder();
        for (int i = 0; i < 6; i++)
            captcha.Append(combination[random.Next(combination.Length)]);
        Session["captcha"] = captcha.ToString();
        imgCaptcha.ImageUrl = "~/Captcha/GenerateCaptcha.aspx?" + DateTime.Now.Ticks.ToString();
    } catch
    {
        throw;
    }
}

Step 5: On click of refresh button call function FillCapctha().