[localhost] Using reCAPTCHA on localhost

It's so easy:

1. Go to your google reCaptcha admin panel
2. Add localhost & 127.0.0.1 to domains of a new site like the following image.

Update:

If your question is how to set reCaptcha in Google site for using it in localhost, then i has been wrote it above but if you are curious that how you can using reCAPTCHA on both localhost and website host by minimal codes in your controller and prevent some codes like ConfigurationManager.AppSettings["ReCaptcha:SiteKey"] in it then I help you with this extra description and codes in my answer.

Do you like the following GET and POST actions?

It support reCaptcha and doesn't need any other codes for handling reCaptcha.

[HttpGet]
[Recaptcha]
public ActionResult Register()
{
    // Your codes in GET action
}

[HttpPost]
[Recaptcha]
[ValidateAntiForgeryToken]
public ActionResult Register(RegisterViewModel model, string reCaptcha_SecretKey){
   // Your codes in POST action
   if (!ModelState.IsValid || !ReCaptcha.Validate(reCaptcha_SecretKey))
   {
       // Your codes
   }
   // Your codes
}

In View: (reference)

@ReCaptcha.GetHtml(@ViewBag.publicKey)

@if (ViewBag.RecaptchaLastErrors != null)
{
    <div>Oops! Invalid reCAPTCHA =(</div>
}

To use it

A) Add the following ActionFilter to your Web project:

public class RecaptchaAttribute : FilterAttribute, IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext filterContext)
    {
        var setting_Key = filterContext.HttpContext.Request.IsLocal ? "ReCaptcha_Local" : "ReCaptcha";
        filterContext.ActionParameters["ReCaptcha_SecretKey"] = ConfigurationManager.AppSettings[$"{setting_Key}:SecretKey"];
    }

    public void OnActionExecuted(ActionExecutedContext filterContext)
    {
        var setting_Key = filterContext.HttpContext.Request.IsLocal ? "ReCaptcha_Local" : "ReCaptcha";
        filterContext.Controller.ViewBag.Recaptcha = ReCaptcha.GetHtml(publicKey: ConfigurationManager.AppSettings[$"{setting_Key}:SiteKey"]);
        filterContext.Controller.ViewBag.publicKey = ConfigurationManager.AppSettings[$"{setting_Key}:SiteKey"];
    }
}

B) Add the reCaptcha settings keys for both localhost & website like it in your webconfig file:

<appSettings>

    <!-- RECAPTCHA SETTING KEYS FOR LOCALHOST -->
    <add key="ReCaptcha_Local:SiteKey" value="[Localhost SiteKey]" />
    <add key="ReCaptcha_Local:SecretKey" value="[Localhost SecretKey]" />
    <!-- RECAPTCHA SETTING KEYS FOR WEBSITE -->
    <!--<add key="ReCaptcha:SiteKey" value="[Webite SiteKey]" />
    <add key="ReCaptcha:SecretKey" value="[Webite SecretKey]" />-->

    <!-- OTHER SETTING KEYS OF YOUR PROJECT -->

</appSettings>

Note: By this way you did not need set reCaptcha_SecretKey parameter in the post action or any ViewBag for reCaptcha manually in your Actions and Views, all of them will be filled automatically at runtime with appropriate values depending on you have run the project on the localhost or website.