controlleur.cs

[HttpPost]
public async Task<ActionResult> Form()
{
    RecaptchaVerificationHelper recaptchaHelper = this.GetRecaptchaVerificationHelper();

    if (String.IsNullOrEmpty(recaptchaHelper.Response))
    {
        ModelState.AddModelError("", "Le captcha ne peut être vide.");
        return PartialView();
    }

    RecaptchaVerificationResult recaptchaResult = await recaptchaHelper.VerifyRecaptchaResponseTaskAsync();

    if (recaptchaResult != RecaptchaVerificationResult.Success)
    {
        ModelState.AddModelError("", "Réponse captcha invalide.");
        return PartialView();
    }

    if (ModelState.IsValid)
    {
        return PartialView("_FormResult");
    }

    return PartialView();
}