RijndaelManaged類是.NET框架中用于對稱加密和解密數據的類。下面是一個簡單的示例，演示如何使用RijndaelManaged類進行數據加密和解密：

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

public class Program
{
    static void Main()
    {
        string plainText = "Hello, world!";
        string key = "0123456789abcdef";
        string iv = "fedcba9876543210";

        byte[] encryptedData = EncryptData(plainText, key, iv);
        string decryptedText = DecryptData(encryptedData, key, iv);

        Console.WriteLine("Encrypted Data: " + Convert.ToBase64String(encryptedData));
        Console.WriteLine("Decrypted Text: " + decryptedText);
    }

    static byte[] EncryptData(string plainText, string key, string iv)
    {
        using (RijndaelManaged rijndael = new RijndaelManaged())
        {
            rijndael.Key = Encoding.UTF8.GetBytes(key);
            rijndael.IV = Encoding.UTF8.GetBytes(iv);

            ICryptoTransform encryptor = rijndael.CreateEncryptor(rijndael.Key, rijndael.IV);

            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                {
                    byte[] plainBytes = Encoding.UTF8.GetBytes(plainText);
                    cs.Write(plainBytes, 0, plainBytes.Length);
                }

                return ms.ToArray();
            }
        }
    }

    static string DecryptData(byte[] encryptedData, string key, string iv)
    {
        using (RijndaelManaged rijndael = new RijndaelManaged())
        {
            rijndael.Key = Encoding.UTF8.GetBytes(key);
            rijndael.IV = Encoding.UTF8.GetBytes(iv);

            ICryptoTransform decryptor = rijndael.CreateDecryptor(rijndael.Key, rijndael.IV);

            using (MemoryStream ms = new MemoryStream(encryptedData))
            {
                using (CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader sr = new StreamReader(cs))
                    {
                        return sr.ReadToEnd();
                    }
                }
            }
        }
    }
}

在上面的示例中，我們定義了EncryptData和DecryptData兩個方法來分別進行數據的加密和解密。在Main方法中，我們演示了如何使用這兩個方法來加密和解密數據。請注意，需要提供一個密鑰和初始化向量（IV）來進行加密和解密操作。