How to implement reCaptcha V3 in ASP.NET

The simplest implementation:

1. In your cshtml file (at the top)

   @section Scripts
   {
       <script src="https://www.google.com/recaptcha/api.js?render=your site key"></script>
       <script>
           grecaptcha.ready(function () {
               grecaptcha.execute('your site key', { action: 'homepage' }).then(function (token) {
                   document.getElementById("foo").value = token;
               });
           });
       </script>
   }
   

2. In your cshtml, inside the form (just before </form>):

   <input type="hidden" id="foo" name="foo" />
   

3. A function inside your Pagemodel class. See the documentation for the response object:

   public static bool ReCaptchaPassed(string gRecaptchaResponse)
   {
       HttpClient httpClient = new HttpClient();
   
       var res = httpClient.GetAsync($"https://www.google.com/recaptcha/api/siteverify?secret=your secret key no quotes&response={gRecaptchaResponse}").Result;
   
       if (res.StatusCode != HttpStatusCode.OK) 
       {
           return false;
       }
       string JSONres = res.Content.ReadAsStringAsync().Result;
       dynamic JSONdata = JObject.Parse(JSONres);
   
       if (JSONdata.success != "true" || JSONdata.score <= 0.5m)
       {
           return false;
       }
   
       return true;
   }
   

4. Finally, inside your OnPostAsync() handler, at the top:

   if (!ModelState.IsValid) 
   {
       return Page();
   }
   else
   {
       if (!ReCaptchaPassed(Request.Form["foo"]))
       {
           ModelState.AddModelError(string.Empty, "You failed the CAPTCHA.");
           return Page();
       }
   }