How to validate google recaptcha on server side in asp.net (c#)?

You can use "IsValidCaptcha()" method to validate your google recaptcha on server side. Replace your secret key with "YourRecaptchaSecretkey" in the following method.

  
Public bool IsValidCaptcha()
 {
  string resp = Request["g-recaptcha-response"];
  var req = (HttpWebRequest)WebRequest.Create
            (https://www.google.com/recaptcha/api/siteverify?secret=+ YourRecaptchaSecretkey + "&response=" + resp);
     using (WebResponse wResponse = req.GetResponse()) 
       {
       using (StreamReader readStream = new StreamReader(wResponse.GetResponseStream()))
         {
          string jsonResponse = readStream.ReadToEnd();
          JavaScriptSerializer js = new JavaScriptSerializer();
          // Deserialize Json
          CaptchaResult data = js.Deserialize<CaptchaResult>(jsonResponse); 
            if (Convert.ToBoolean(data.success))
              {
               return true;
              }
         }
      }
     return false;
 }

Also you need to create following class as well.

public class CaptchaResult
  {
   public string success { get; set; }
  }

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