Simple method to avoid comment spam

We probably all know about the annoying captcha images that a lot of blogs uses for separating humans from machines (spam robots). I use a captcha image to avoid comment spam on this blog because I get a lot, but I really don’t like to use it. I don’t like the fact that it makes is more difficult for my visitors to write comments, which is my only way of measuring the quality of the individual posts.

What I want is an invisible unobtrusive captcha method that automatically makes sure the user is human. So I wrote a simple method that does just that. It works by adding a small JavaScript to the page that adds a hidden form field when the form is submitted. The value of the hidden field must be the same as a server-side variable to validate. There must also be a property that returns a Boolean value that indicates whether or not the user is human.

/// <summary>

/// Initializes the captcha and registers the JavaScript

/// </summary>

private void InititializeCaptcha()

{

  if (ViewState["captchavalue"] == null)

  {

    ViewState["captchavalue"] = Guid.NewGuid().ToString();

  }

 

  System.Text.StringBuilder sb = new System.Text.StringBuilder();

  sb.AppendLine("function SetCaptcha(){");

  sb.AppendLine("var form = document.getElementById('" + Page.Form.ClientID + "');");

  sb.AppendLine("var el = document.createElement('input');");

  sb.AppendLine("el.type = 'hidden';");

  sb.AppendLine("el.name = 'captcha';");

  sb.AppendLine("el.value = '" + ViewState["captchavalue"] + "';");

  sb.AppendLine("form.appendChild(el);}");

 

  Page.ClientScript.RegisterClientScriptBlock(GetType(), "captchascript", sb.ToString(), true);

  Page.ClientScript.RegisterOnSubmitStatement(GetType(), "captchayo", "SetCaptcha()");

}

 

/// <summary>

/// Gets whether or not the user is human

/// </summary>

private bool IsCaptchaValid

{

  get

  {

    if (ViewState["captchavalue"] != null)

    {

      return Request.Form["captcha"] == ViewState["captchavalue"].ToString();

    }

 

    return false;

  }
}

Examples of use

To use the captcha you have to call InitializeCaptcha from the Page_Load handler. Then just check the IsCaptchaValid property before you save the comment.

protected void Page_Load(object sender, EventArgs e)

{

  InititializeCaptcha();

}

 

/// <summary>

/// Handles the submit buttons onclick event

/// </summary>

void btnSave_Click(object sender, EventArgs e)

{

  if (IsCaptchaValid)

  {

    SaveComment();

  }
}