No CAPTCHA reCAPTCHA for ASP.NET WebForms

The code below is an example approach for reCAPTCHA implemented as ASP.NET WebForms control:

/// <summary>

/// NoCaptcha ReCaptcha for WebForms

/// </summary>

/// <remarks>

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

/// </remarks>

public class NoRecaptchaControl : Control

{

    public class APIResponse

    {

        public bool success { get; set; }

        public string[] error_codes { get; set; }

    }

    public string PrivateKey { get; set; }

    public string PublicKey { get; set; }

    public string Theme { get; set; }

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

    public bool IsValid

    {

        get

        {

            try

            {

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

                query["secret"]   = this.PrivateKey;

                query["response"] = this.Context.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;

                return response.success;

            }

            catch ( Exception ex )

            {                

                return false;

            }

        }

    }

    protected override void Render( HtmlTextWriter writer )

    {

        writer.WriteBeginTag( "script" );

        writer.WriteAttribute( "src", "https://www.google.com/recaptcha/api.js" );

        writer.WriteAttribute( "async", "true" );

        writer.WriteAttribute( "defer", "true" );

        writer.Write( HtmlTextWriter.TagRightChar );

        writer.WriteEndTag( "script" );

        writer.WriteBeginTag( "div" );

        writer.WriteAttribute( "class", "g-recaptcha" );

        writer.WriteAttribute( "data-sitekey", this.PublicKey );

        writer.Write( HtmlTextWriter.TagRightChar );

        writer.WriteEndTag( "div" );

        base.Render( writer );

    }

}

This uses the HttpClient, make sure to reference the Microsoft.AspNet.WebApi.Client package.

The usage is simple, just put the control on your form:

<%@ Register TagPrefix="recaptcha" Namespace="Recaptcha" Assembly="My.Recaptcha.Assembly.Name" %>

...

<recaptcha:NoRecaptchaControl 

  ID="recaptcha" runat="server" 

  ClientIDMode="Static" Theme="clean" />

set both private and public keys:

protected override void OnPreInit( EventArgs e )

{

    base.OnPreInit( e );

    recaptcha.PrivateKey         = "...";

    recaptcha.PublicKey          = "...";

}

and you are done:

protected void button_Click( object sender, EventArgs e )

{

    if ( recaptcha.IsValid )

    {

        ...

    }

    else

    {

        ...

    }

}