string stringA = "appid=wxd930ea5d5a258f4f&body=test&device_info=1000&mch_id=10000100&nonce_str=ibuaiVcKdpRxkhJA";

            string stringSignTemp = stringA + "&key=192006250b4c09247ec02edce69f6a2d";

            string sign = MD5Helper.GetMD5Hash(stringSignTemp).ToUpper();

 public class MD5Helper
    {
        public static string GetMD5Hash(string value)
        {
            MD5 md5 = new MD5CryptoServiceProvider();
            byte[] hashByte = md5.ComputeHash(Encoding.Default.GetBytes(value));
            StringBuilder sb = new StringBuilder();
            foreach (byte b in hashByte)
            {
                sb.Append(b.ToString("x").PadLeft(2, '0'));
            }
            return sb.ToString();
        }
    }