Verify ReCaptcha responses server-side.

ReCaptcha.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;

namespace Antiyes
{
    public class ReCaptcha
    {
        public static async System.Threading.Tasks.Task<SiteVerifyResponse> VerifyUserResponse(string secret, string response,
            string remoteip = null, bool throwOnError = false, string verifyUrl = "https://www.google.com/recaptcha/api/siteverify")
        {
            using (var client = new HttpClient())
            {
                var values = new Dictionary<string, string> { { "secret", secret }, { "response", response } };
                if (remoteip != null)
                {
                    values.Add("remoteip", remoteip);
                }
                var postData = new FormUrlEncodedContent(values);
                var postResponse = await client.PostAsync(verifyUrl, postData);
                var responseString = await postResponse.Content.ReadAsStringAsync();
                var verifyResponse = JsonConvert.DeserializeObject<SiteVerifyResponse>(responseString);

                if (throwOnError)
                {
                    if (verifyResponse.Errors != null && verifyResponse.Errors.Length > 0)
                    {
                        var errors = string.Join(", ", verifyResponse.Errors.Select(a => ErrorCodes[a]));
                        throw new ReCaptchaVerifyException(errors);
                    }
                }

                return verifyResponse;
            }
        }

        public static Dictionary<string, string> ErrorCodes => new Dictionary<string, string>  {
            { "missing-input-secret","The secret parameter is missing."},
            { "invalid-input-secret","The secret parameter is invalid or malformed."},
            { "missing-input-response","The response parameter is missing."},
            { "invalid-input-response", "The response parameter is invalid or malformed."}
        };

        public class ReCaptchaVerifyException : Exception
        {
            public ReCaptchaVerifyException() { }
            public ReCaptchaVerifyException(string message) : base(message) { }
            public ReCaptchaVerifyException(string message, Exception inner) : base(message, inner) { }
        }

        public class SiteVerifyResponse
        {
            [JsonProperty(PropertyName = "success")]
            public bool Success { get; set; }
            [JsonProperty(PropertyName = "challenge_ts")]
            public DateTime ChallengeTimestamp { get; set; }
            [JsonProperty(PropertyName = "hostname")]
            public string Hostname { get; set; }
            [JsonProperty(PropertyName = "error-codes")]
            public string[] Errors { get; set; }
        }
    }
}