No CAPTCHA reCAPTCHA for ASP.NET MVC

This time the implementation of the reCAPTCHA for ASP.NET MVC. We need an HTML helper:

/// <summary>

/// Extension dla recaptcha

/// </summary>

/// <remarks>

/// https://developers.google.com/recaptcha/docs/display

/// </remarks>

public static class CaptchaHtmlHelper

{

    // private and public keys are read from appsettings but

    // feel free to modify it so that is suits your needs

    private static string CaptchaPrivateKey

    {

        get

        {

            return ConfigurationManager.AppSettings["recaptchaPrivate"];

        }

    }

    private static string CaptchaPublicKey

    {

        get

        {

            return ConfigurationManager.AppSettings["recaptchaPublic"];

        }

    }

    public static MvcHtmlString GenerateCaptcha(

        this HtmlHelper htmlHelper )

    {

        TagBuilder script = new TagBuilder( "script" );

        script.MergeAttribute( "src", "https://www.google.com/recaptcha/api.js?hl=pl" );

        script.MergeAttribute( "async", "true" );

        script.MergeAttribute( "defer", "true" );

        TagBuilder div = new TagBuilder( "div" );

        div.MergeAttribute( "class", "g-recaptcha" );

        div.MergeAttribute( "data-sitekey", CaptchaPublicKey );

        return new MvcHtmlString( 

           script.ToString( TagRenderMode.Normal ) + 

           div.ToString( TagRenderMode.Normal ) );

    }

}

and an auxiliary validation attribute

/// <summary>

/// Captcha validation

/// </summary>

/// <remarks>

/// https://developers.google.com/recaptcha/docs/verify

/// </remarks>

public class CaptchaValidatorAttribute : ActionFilterAttribute

{

    public class APIResponse

    {

        public bool success { get; set; }

        public string[] error_codes { get; set; } 

    }

    private const string RESPONSE_FIELD_KEY = "g-recaptcha-response";

    public override void OnActionExecuting( ActionExecutingContext filterContext )

    {

        try

        {

            var query = HttpUtility.ParseQueryString( string.Empty );

            query["secret"] = ConfigurationManager.AppSettings["recaptchaPrivate"];

            query["response"] = filterContext.HttpContext.Request.Form[RESPONSE_FIELD_KEY];

            HttpClient client = new HttpClient();

            client.BaseAddress = new Uri( "https://www.google.com/" );

            // Add an Accept header for JSON format.

            client.DefaultRequestHeaders.Accept.Add( new MediaTypeWithQualityHeaderValue( "application/json" ) );

            HttpResponseMessage httpresponse = 

                client.GetAsync( string.Format( "recaptcha/api/siteverify?{0}", query.ToString() ) ).Result;

            var response = httpresponse.Content.ReadAsAsync<APIResponse>().Result;

            // this will push the result value into a parameter in our Action  

            filterContext.ActionParameters["captchaValid"] = response.success;

            base.OnActionExecuting( filterContext );

        }

        catch ( Exception ex )

        {

            filterContext.ActionParameters["captchaValid"] = false;                

        }

    }

}  

The validation attribute uses the HttpClient, make sure you reference the Microsoft.AspNet.WebApi.Client package.

To use the extension, place it in your view:

<div> 

    @Html.GenerateCaptcha()

</div>

and decorate your action with the validation attribute:

[HttpPost]

[CaptchaValidator]

public ActionResult TheActionName( bool captchaValid )

{

    if ( ModelState.IsValid && captchaValid )

    {

        ...

As you can see the validation attribute is responsible for setting the value of the captchaValid parameter and the value is then passed down to the action decorated with the attribute.