public static string DecodeUserInfo(string encryptedData, string iv, string code)
        {
            var url = "https://api.weixin.qq.com/sns/jscode2session?appid=微信AppID&secret=开发秘钥&js_code=" + code + "&grant_type=authorization_code";
            var sessionkey = Utils.HttpGet(url);
            var wxinfo = Newtonsoft.Json.JsonConvert.DeserializeObject<WxInfoRequest>(sessionkey);
            var session_key = wxinfo.session_key;
 
            byte[] iv2 = Convert.FromBase64String(iv);
 
            if (string.IsNullOrEmpty(encryptedData)) return "";
            Byte[] toEncryptArray = Convert.FromBase64String(encryptedData);
 
            System.Security.Cryptography.RijndaelManaged rm = new System.Security.Cryptography.RijndaelManaged
            {
                Key = Convert.FromBase64String(session_key),
                IV = iv2,
                Mode = System.Security.Cryptography.CipherMode.CBC,
                Padding = System.Security.Cryptography.PaddingMode.PKCS7
            };
 
            System.Security.Cryptography.ICryptoTransform cTransform = rm.CreateDecryptor();
            Byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);
 
            return Encoding.UTF8.GetString(resultArray);
        }

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace YouNameSpace
{
    using Newtonsoft.Json;
    using System;
    using System.Collections.Generic;
    using System.IO;
    using System.Linq;
    using System.Security.Cryptography;
    using System.Text;
    /// <summary>
    /// 签名及加密帮助类
    /// </summary>
    public static class EncryptHelper
    {

        #region 私有方法

        private static byte[] AES_Decrypt(String Input, byte[] Iv, byte[] Key)
        {
            RijndaelManaged aes = new RijndaelManaged();
#if NET45
            
#else
            //SymmetricAlgorithm aes = Aes.Create();
#endif
            aes.KeySize = 128;//原始：256
            aes.BlockSize = 128;
            aes.Mode = CipherMode.CBC;
            aes.Padding = PaddingMode.PKCS7;
            aes.Key = Key;
            aes.IV = Iv;
            var decrypt = aes.CreateDecryptor(aes.Key, aes.IV);
            byte[] xBuff = null;
            using (var ms = new MemoryStream())
            {
                using (var cs = new CryptoStream(ms, decrypt, CryptoStreamMode.Write))
                {
                    byte[] xXml = Convert.FromBase64String(Input);
                    byte[] msg = new byte[xXml.Length + 32 - xXml.Length % 32];
                    Array.Copy(xXml, msg, xXml.Length);
                    cs.Write(xXml, 0, xXml.Length);
                }
                xBuff = decode2(ms.ToArray());
            }
            return xBuff;
        }

        private static byte[] decode2(byte[] decrypted)
        {
            int pad = (int)decrypted[decrypted.Length - 1];
            if (pad < 1 || pad > 32)
            {
                pad = 0;
            }
            byte[] res = new byte[decrypted.Length - pad];
            Array.Copy(decrypted, 0, res, 0, decrypted.Length - pad);
            return res;
        }

        #endregion

        /// <summary>
        /// 解密所有消息的基础方法
        /// </summary>
        /// <param name = "sessionKey" > 储存在 SessionBag 中的当前用户 会话 SessionKey</param>
        /// <param name = "encryptedData" > 接口返回数据中的 encryptedData 参数</param>
        /// <param name = "iv" > 接口返回数据中的 iv 参数，对称解密算法初始向量</param>
        //    / <returns></returns>
        public static string DecodeEncryptedData(string sessionKey, string encryptedData, string iv)
        {
            var aesCipher = Convert.FromBase64String(encryptedData);
            var aesKey = Convert.FromBase64String(sessionKey);
            var aesIV = Convert.FromBase64String(iv);

            var result = AES_Decrypt(encryptedData, aesIV, aesKey);
            var resultStr = Encoding.UTF8.GetString(result);
            return resultStr;
        }
    }
}