string keyString = "encrypt123456789";
var key = Encoding.UTF8.GetBytes(keyString);//16 bit or 32 bit key string

using (var aesAlg = Aes.Create())
{
    using (var encryptor = aesAlg.CreateEncryptor(key, aesAlg.IV))
    {
        using (var msEncrypt = new MemoryStream())
        {
            using (var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
            using (var swEncrypt = new StreamWriter(csEncrypt))
            {
                swEncrypt.Write(text);
            }

            var iv = aesAlg.IV;

            var decryptedContent = msEncrypt.ToArray();

            var result = new byte[iv.Length + decryptedContent.Length];

            Buffer.BlockCopy(iv, 0, result, 0, iv.Length);
            Buffer.BlockCopy(decryptedContent, 0, result, iv.Length, decryptedContent.Length);

            return Convert.ToBase64String(result);
        }
    }
}